motionbuilder is real-time animation...with maya it slows down a lot when it has to calculate each keyframe, geometry, and everything else...motionbuilder is so smooth, it's amazing...aaaaaaaaaand...you can keyframe on layers to make things easier...i've been checking out a few of the features and they're really cool...it's also got full body IK, which i thing maya 7 has in some capacity, but i'm not sure...it's at least worth looking into as you can simulate a real production pipeline

plus...maya isnt just for animation while motionbuilder is...which means it's much more robust than maya when it comes to animation