Neither will be really successful and I doubt Sony at least is banking much on it. ( MSFT seems to do a bit too much communication around Natal and are setting themselves for a huge disappointment if it fails).
The main reason both won't encounter much success is that the majority of the software on either platforms won't support it( even if software started to support those once they are released, the defining games on each platform won't support it , be it GTA4, Halo3, Uncharted 2, Cod4 and MF2 and so on...)
As for getting ports, there aren't that many Wii third party titles that are begging for a port to HD consoles ( aside maybe from EA Sports Active but you need a balance board, not Natal or Sphere).
PS3-Xbox360 gap : 1.5 millions and going up in PS3 favor !
PS3-Wii gap : 20 millions and going down !
