R.J.
xBox360.
Hands down.
WHY?
Well, not only because we own one... but because we spend 6mo in and out of children's hospital this year. Why is THAT a point? Because they give Wii's to the bedbound children to use. All you need to do to use them is to flick your wrist. Yes they SAY to stand and crouch, or swing your whole arm... but it is VERY easy to cheat. Which is why sick kids who can barely move can use them for hours.
Even with the pads (expensive, and they break) for the floor... It's VERY easy to cheat. Really, really easy.
The 360, otoh, is filming your entire body. It will correct you if your hand is facing out instead of in, or if your shoulders (or body, hips, etc.) are at the wrong angle.
Many of the programs are also 'settable' for injuries or conditions. So you can do yoga or pilates with all the ones that would stress your injury taken out. The dance games catch if you're not popping your shoulders, or just stepping straight instead of stepping with a twist. AWESOME. And there's no lag.
It rains here apx 10mo a year. The 360 is PHENOM for at home / indoor exercise. The Wii is fun, but nowhere near as good, and all the controllers and pads are PRICEY (several hundred)