I can't see either Sony or Microsoft succeeding, regardless of the quality of their actual products. The problem is that the "hardcore" gamers, the ones who flock to the Playstation and Xbox, don't want a motion controller. They're content to play games with a gamepad. Meanwhile, the more "casual" gamers have already gone with the Wii. Anybody looking into buying a new system with motion controls will see that the Wii is $200 while the PS3/360 are ~$300 plus the cost of the motion controllers.
This has nothing to do with the quality of the products (I suspect Natal's going to suck hard, though the feedback from Move seems to be pretty good) and everything to do with what people want. If Sony or Microsoft released a new, cheaper console with a motion controller packed in, I could see that being successful. As it stands, I can't see too many people dropping $100 to play LittleBigPlanet with a motion controller.
Natural laws have no pity.