I believe both platforms have their advantages and disadvantages. In my opinion, I find XenForo to have pretty good basic features, gives you a lot of room for customizations. But has poor support. Although not related to XenForo developers, add-ons are ridiculously expensive As for IPB4 has a lot of neat features, but lacks customization I find. And prices are high, they charge you for pretty much everything.
Remember the good old days when all hosts ran a forum? We had one that had 100,000 posts and ran vBulletin. VB was king back then but then they changed their pricing model and pretty much killed their business.
I manage various communities and in terms of core functionality, XenForo 2 wins over IPB in my opinion. IPB doesn't feel as well organized as XenForo. For example, the only way to ban a user appears to be via the admin CP, there's no front-end option like there is on XenForo, MyBB, phpBB, etc. Feel free to correct me if I'm wrong here, maybe I just haven't seen the option yet. IPB does have a lot of strengths though - it's really nice to customize (I love the "Blocks" feature). They're both great platforms, but XenForo is my favourite right now.