I choose vBulletin 5 because it's tried and tested, been around for a very long time and not going anywhere soon. The other end is I tried to run sites on software where the people behind it did not have the resources to keep it going, and they ended up stopping development and my sites died because of it, i.e., I had to start again. I think this has happened to me for 3-5 different software wasting a lot of my time and money. Now I'd try to use vBulletin for everything if possible.
I'm think I'm 95% sure vBulletin isn't going anywhere and probably a good 75% sure XenForo isn't going anywhere, so either of those is a good choice, IMO.