I remember XF at beta, and it seemed better developed (obviously, that's a subjective view and I have no way to provide any objective, measurable criteria).
vb5 beta has some interesting features that seem to be unique to vb5 (comment system on posts, insert link feature, and a couple others) but...
1) It is so slow (XF seemed like it made a point to be as quick as possible from the beginning)
2) It doesn't seem to make good use of jquery (eg. XF's member profile is brilliant IMO)
3) It seems like it is trying to rely too much on its activity stream (which to me, seems like a bad attempt at a port of a Facebook like structure)
4) It lacks a lot of features that are very much needed (infraction system, reputation, and other essential features for modern forum platforms - however, they may be added in future releases)
5) The notification system is really, really bad it seems. The only way to remove the notification alert...is to delete the notification itself in your profile (unlike how XF does it via cookie, automatically).
6) Clicking on a notification pulls up a single post...with little to no reference as to where it was made. It is a very confusing system (to me anyway).
It's beta. Many changes are coming, I know. But it isn't intuitive, and IMO, modern forum platforms ought to be user friendly, intuitive, logical, efficient.
If both were to be graded right now, XF would win hands down. But many changes are coming, with many new features to be added.
I prefer XF, but because of the unknown future, I can't migrate my existing vb4 board to XF (despite having an XF licence for over a year). And because I don't trust and like IB (the company who owns vb), I can't rush out and buy the beta release of vb5 (that, and because it just isn't a polished product).
I'm hoping XF can survive the turmoil. XF being further developed and supported is my #1 choice for my communities. But if it isn't, then vb5, despite my reservations against IB, will be #2 assuming that vb5 gets a lot of the kinks worked out.
But as of right now, it has a long road to travel...and it at its beta stage, appears to be behind XF when it was at its beta stage.