I would go for IPS 4.0.x because it way better then the current xenForo, alot more features and a massive upgrade from its 3.4.x sibling which was a resource hog, however xenForo is now stopping development for its 1.x.x series and working on 2.0 series.
Choosing the right forum software comes down to 2 things, which are You & Type of forum, what I mean by this is how well do you know xenForo, do you like it better then IPB. if yes go for it, the other thing is what type for forum do you have, if its a gaming forum I would use xenForo, a professional forum go with IPB.
My option is I would got for IPB, because I know my way around it more then xenForo. But if you can still wait 4-5 months wait what xenForo 2.0 has to offer and then chose from xenForo 2.0 & IPS 4.0
Personally I use IPB 4.0.x and it's awesome, it's user friendly, more light and optimal than the previous versions. But as someone said, wait the coming out of Xenforo 2.0 in few months and decide what's best
We used both of them and to be honest, it's totally depends upon your personal preference. Xenforo's ACP look's kinda dull but is packed with a ton of features.
IP.B have a modern looking ACP with slightly less features but it's totally up to you on what your community needs.