I recommend keeping the latest 3.X.X build and if you ever do have to move on, IPB is the correct route. vBulletin is no longer the same. All the developers are new, every single one of them. 4.0 is horrible. I'm keeping all my sites on 3.X.X and once I can't keep it stable anymore, I'm off to IPB (which I use a lot aswell, it's great). 3.X.X is better than IPB, but once 4.0 comes out, IPB has the win.