I would love to go over the whole story but to make it short, personally I would suggest going over to XenForo from the start. I used to be a long time user of IPB (I followed Matt Mecham since Ikonboard days) and while they do have a great product, I don't think it is what you are looking for HostingDiscussion.com. Their product has gone a little bit further than being a community system to be a whole ecosystem with features that take away from the actual focus of a forum (user discussion).
XenForo is created by the original creators of vBulletin and some new promising staff. They are already on the way for v2 and looking to improve the modding community and make XenForo more flexible and modern, which most forum systems are lacking. If you are unsure, I would suggest waiting for XenForo v2 to take a look and compare with IPB, then decide.
I am in no means associated with XenForo.