IPB is by far the best paid forum software at this current time. Xenforo still has a few features to add and areas to improve before I would consider moving to them.
vB 4.x is probably the worst paid forum software to ever come out, in my opinion.
I can't really tell what will happen in the future however I really would love to see a totally redesigned IPB in the near future. I think the 3.x series has sadly run it's course and needs a swift update to get it up to modern social networking standards. I don't mean copy Facebook and Twitter but make IPB function in a fast and simple way like those sites, would be amazing.
The latest 3.2 release was a step in the right direction I wanted to IPB to go however it still feels "clunky" in places for users. An example of this would be the notifications menu, which I think is totally off putting to users and should never be in a product like IPB. Others are the current editor which still seems to have loads of bugs.
If there will ever be a 4.0 series of IPB then I would like to see a lot of features modernised, condensed and removed. Xenforo might not have the features of IPB but they are building it in the right way by adding features over time, especially when they can keep an eye on sites like Twitter and Facebook. They also have had the advantage of coming out with totally new coded software so they didn't have to put up with complaints like "why did you remove x feature?" etc. IPS don't have that advantage which makes it harder for them to advance.
My suggestion would still be IPB tho.