XenForo or IPB? Which is better?

NulledBB is using MyBB.

XenForo is a nice enough software, but from what I could gather, it is rather limited, yet easy to use.

IPB is easier to customize and I've seen some good things been done with it. Though it also sacrifices freedom for ease-of-use.

Though personally I'd recommend that you use either MyBB or vBulletin.