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.
No surprise that the commercial solution is better It did look more polished.
But its good to know that the free solution also works if needed. It's win / win really.
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.