Which one of these is a better solution?
In terms of index, it does not matter. Both will index your content in same way. It's just XML file, Google doesn't favor one over others. Google also treat XML sitemap,atom,text and rss file in same way. Also sitemap file just help you to index content, it doesn't have any relation with ranking as well.
So the difference is, If you create XML file manually, then it will take your some of time, but by doing that wordpress does not need to create automatically, so it will save some of your server resource, but again it will help to save only 0.1% resource or may be even lesser.