With the new line of mirrorless cameras being announced this fall, the gap between DSLR and mirrorless cameras continue to close. There are many generic discussions comparing the two, but as a portrait photographer, I'm not needing a complex AF tracking (sports), every action with a dedicated button (wedding), super high ISO capabilities for the candlelight situations (wedding again), or +200mm lenses (wildlife).
Like any photographer, portrait photographers want sharp quality, rugged cameras (for on-site portraits), variety of lenses, etc. Portrait photographers particularly like strobe compatibility, wide apertures (bokeh), and possibly tethering capabilities.
As a portrait photographer, what all should a person consider?
A few aspects mentioned in your question will be our starting point. Please note, we are not saying each of these issues will be determining factors for every photographer. We're not saying one system is better than the other because of... a or b. Rather they are a response to the question, "...what all should a person consider?" Once considered, each of these aspects may or may not lead a particular portraitist to choose one system over the other.
I've heard a couple pro photographers talk about how the DSLR commands a certain kind of respect from clients that they wouldn't get with a mirrorless system and I've seen this first hand as well. I was running a 60d w/grip and 70-200 F/2.8 and people thought I was some hotshot pro. So if you want to charge a lot people want to feel they're getting their money's worth and DSLRs help convey quality when they don't know any better.
I don't know much about the current state of the art with mirrorless, but while a lot of things can eventually be compensated for (such as sensor size, PDAF capability (with hybrid AF chips, lens and strobe compatibility, etc), one thing that they can't do is give you an actual direct, through the lens image path.
A mirrorless can only ever show you what the sensor can see, never what is actually there. Anything that extends beyond the range of what the sensor sees won't be visible to you and that can have an impact on shot composition. Starting out, it might be nice to be able to see things how the sensor sees it to know exactly how it will come out, but once you have some experience, it is nice to be able to see what is actually there to know what you are and aren't capturing.
Battery life may also be another factor depending on the shooting environment. Mirrorless has to run the LCD, so it gets much less battery life than a DSLR can get when using the viewfinder, particularly if you get good at not having to spend a lot of time checking photos.
Room for dedicated buttons is also nice. It is just a convenience thing if you have time, but time you don't keep the customer waiting is time that the customer gets to keep and time you can spend working with another client, so it does have a benefit to both you and your customers.
There is also of course the "professional" look of a DSLR that shouldn't be underestimated when it comes to marketing yourself either. If you want to do commercial work, having people think your gear is big and fancy and expensive is almost if not more important than the actual quality and cost of your gear. This gap may also close, but having customers feel like they are getting their monies worth and that it isn't something they could have done with a point and shoot and a tripod is valuable for customer satisfaction.
The advantages of a DSLR over a mirrorless interchangeable lens camera (MILC) for portraiture are starting to vanish, with the latest releases it's pretty much limited to:
Optical viewfinder is sharper, better in contrasty light and low light, offers instant feedback and better battery life.
Ergonomics are usually better, especially for larger lenses.
Compatibility with flash systems and tethering.
Most of the other factors cited as advantages of DSLRs are now moot:
Image quality. Full frame mirrorless cameras from Sony promise as good or better image quality than the best DSLRs.
Lens quality / bokeh. Mirrorless bodies allow the widest selection of lenses from different formats to be adapted, in some cases (Metabones for Canon AF) allowing AF and aperture control from the camera body. Adaptor horror stories courtesy of LensRentals etc. mostly apply to wide angle infinity focus use cases, not portraiture.
There are some advantages to mirrorless bodies as well. In addition to lens adaptability, contrast detect autofocus using the main sensor has been shown to be far more consistent and accurate than phase detection in DSLRs, at least in good light. And features such as eye detection (the next step from face detection) take the trial and error out of shallow DOF headshots.
As for the question of "looking professional" a big lens always helps and a more compact body will make the lens appear bigger. I think provided you have a viewfinder you should be all right a vertical grip is helpful too. As is how you act, having to check the focus using the rear screen after every frame isn't going to inspire confidence in the client.
After now spending a few months having switched from DSLR to Mirrorless, I thought I'd provide my experience to this question.
Bokeh / DOF (M43)
The most noticeable difference I have found was shallow DOF. Having switched from a 1.6x crop to a 2x crop without having wider apertures available, I'm now using compression to accomplish the same DOF.
As an example, I previous used a 50mm f/1.4 on a 1.6x crop body, giving me the equivalent of 80mm f/2.24. Looking through my images, I found myself averaging f/2.0 with this lens. After having switched, I'm now using a 75mm f/1.8 and yielding slightly better results in areas I'm able to back further away from the subject. In tight quarters, I'm simply not as able to accomplish the same DOF, but I'm getting enough subject separation to not distract the viewer with the background.
Note: I say M43 because there are Mirrorless FF cameras available where this issue does not apply.
The largest disadvantage to mirrorless is in lower light when the camera doesn't know which direction to start focusing and it starts in the wrong direction. While it's only a split second, it's enough to miss the shot if you are photographing fast action. I have not felt disadvantaged by this when doing portraiture.
Handling / Build
Mirrorless cameras, like DSLR cameras, come in a wide variety of sizes, builds, and UI differences. While this is preference, I found a better selection of handling and UI with the mirrorless selection. While it is a smaller body to work with, the button placement feels very natural and intuitive.
Mirrorless build offers the same weather sealing as many of the top brands. I have tested my mirrorless camera in the rain and extreme cold without issues. Unless you plan to spend $6k+ on a DSLR, the same build quality is available between DSLR and mirrorless.
This is a hot topic for many photographers as the viewfinder is electronic (EVF) as opposed to optical. I was very uncertain about the changes, but have found it to be a improvement for my uses.
One of my favorite features as a portrait photographer is being able to see real-time exposure compensation in the viewfinder. You can expose to the subjects face and watch your exposure adjustments in real time. You can also view the histogram in the EVF before you take the shot. During portrait shoots, I also have a preview appear in the EVF without having to review on the back of the screen. This saves time during the shoot, reduces chimping, and reduces post production adjustments as well.
I haven't found any issues with low-light as the camera boosts the image in the EVF to assist composition.
Having focus peaking or magnification in the viewfinder has resulted in less trips back and forth between the back panel and viewfinder. I'm also able to preview an image in the EVF on very bright days instead of covering the back screen to be able to see the image.
Lens / 3rd party Accessory Selection
This depends heavily on the brand of mirrorless. If you move toward the M43 mirrorless brand, you will find a large selection of lenses in amateur and pro category. I have found that these lenses I need are all available in professional quality. As a portrait photographer, the M43 75mm 1.8 and 45mm 1.8 are an excellent pair.
I wouldn't be able to speak for 3rd party accessories as I no longer have a need for them. There are less available, but you would need to consult your practice and the ability to meet your needs.
Much like DSLR cameras the quality of the camera dictates the features available, but mirrorless is able to optically control up to 4 groups of strobes remotely from the camera body. This was greater than the 3 groups available on my Canon 7D.
M43 seems to peak at 16MP. Due to sensor size, you will be limited. With high-end glass, you're able to crop images and still maintain print quality. Compared to APS-C bodies, you will yield similar results in print, but have less ability to crop if needed.
A very interesting topic has been professional appearance with a smaller camera. While I use one of the larger mirrorless bodies, it is still significantly smaller than DSLR bodies and the lenses are much smaller. The only noticeable difference is that my clients appear to be actually more comfortable in front of the camera. They are able to see more of my face, my emotions, smile, etc. and it reduces the "social distance" between the client and the photographer. Clients are also less concerned about people's reaction in public as well.
In times that I need to appear more professional, like an event, I keep a staff badge in my backpack. This keeps others from jumping out in front of me or glaring when I'm somewhere spectators shouldn't be. Another helpful piece to garner more respect is that I sometimes still use my large DSLR backpack that gives the appearance of a lot of equipment. No one needs to know that it's only half full and 1/4 the weight.
After having switched for a few months, I'm overall very pleased with the decision. While I still gripe about the lacking DOF, I'm able to accomplish the DOF that fits my style. This may not be the same case for you and I cannot stress this enough. It is a gap that cannot be closed (DOF joke intended).
As for the size, weight, EVF, lens selection, professional features, etc. I'm very pleased.
As a portrait photographer, what all should a person consider?
While three years ago, the differences were larger between mirrorless and dSLR cameras, these days, the gap is narrowing. Resolution, format size, lens selections, weatherproofing, etc. are all coming closely into parity. And while fast tracking autofocus still remains on the dSLR side of the line, that's not typically a feature that matters to a portrait photographer who shoots posed subjects. Handling and bulk/weight, of course, are additional big differences, but the decision on their importance relies more on personal preferences.
But there is still one difference that can have a huge effect to a portrait photographer, and that's off-camera lighting capabilities and 3rd-party support. Since lighting is often the distinguishing mark of a professional portrait shooter, this could make a big difference, if you need more than just simple manual-only triggering or want remote power control over both speedlights and studio strobes.
While most of the mirrorless systems also have full TTL infrared systems, like Nikon's CLS/Canon's wireless eTTL (e.g., micro four-thirds's RC system, Sony's wireless flash), radio triggering is a different story.
Both Canon and Nikon now offer RF triggering for remote hotshoe flashes with their OEM gear. And nearly all the third-party TTL/HSS-capable RF flash triggering systems support Nikon and Canon (although Canon does seem to get reverse engineered more often and more quickly by cheap Chinese brands like Yongnuo). And some of those systems may offer Sony-compatible triggering. But the Sony e-mount system is split over two different hotshoe standards (the old Minolta proprietary hotshoe and the newer multi-interface hotshoe) which can complicate matters. And it's still very difficult to find TTL/HSS-capable triggers for Pentax, (micro) four-thirds, and Fuji X.
If having HSS capability, remote group/zoom/power control, or access to flash functions and settings from the camera back is important to you (and the convenience of having this control is something you do get used to and don't want to give up), it can be extraordinarily annoying to move to a system like Fuji X, where HSS has only just been added to the hotshoe protocol with the XT2 and EF-X500. There's no third-party support for Fuji X TTL with triggers. And triggering systems that do Sony and four-thirds HSS/TTL are thin on the ground; let alone studio strobes and barebulb lights as well as speedlights that do HSS/TTL in concert with those systems.
3rd party support is nearly all for Canon and Nikon, both with flashes and triggering systems to support them, not to mention little things like TTL sync cables.
Mirrorless still has a ways to go to catch up when it comes to studio lighting.