I'm playing as a Demon Hunter, mostly solo'ing to this point. I just acquired a Templar as the NPC in my party and have a choice of a first skill. The choices are
Which is the better ability of the two given my style of play?
I had a similar thought when I was leveling, so I'll note first that you can try both out - you can retrain followers through an unfortunately non-obvious method.
That aside, I tried both out and my thoughts are that if you are in a situation where you need a heal on top of blood globes and health potions, you're taking sufficient damage that you're probably not going to stay alive anyway.
The Taunt ability, on the other hand, causes the monsters to run to the Templar, giving you range and more time to set yourself up defensively. It also gives you more of a chance to escape if you choose to run away.
Overall I've chosen the Taunt ability as I find it more useful for my play style. YMMV, but as I mentioned initially you're able to swap and change as often as you want.