I made a concerted effort both to set up, operate, and use both test forums for the past week and I definitely prefer the XenForo one, in terms of completeness, its general polish level, some nice features in the staff-end, and its REST API (which I can use for other purposes in the future). PHPBB is a solid system, but the migration was a lot more difficult and error-prone, and I had to do a lot more work to get a functional copy of OF running under it.
I've been over some of the styles and created some new variants based off the more commonly-used styles available on Orbiter Forum today. They're unfinished, but a good start point for developing a similar look and feel while retaining the more modern HTML-5 layout XenForo provides by default.
I'm also enumerating everything I'd need to do in order to actually complete the work to move the forum over to this software.