Which add-on will sell the most in 2010?

[CommonMan]

Just as the title says, provided all of these come out in 2010 which do you think will sell the most?

I think Natal's success depends on the software. If it has great software support, it could blow the doors off of the competition. If not, dead.

 Sony's Gem could do well, but I'm not sure about the adoption rate of current owners. Also cost could be a limiting factor as well.

I honestly think that the vitality sensor can sell the most. The Wii has somehow maintained that "cool" factor, with Wii-Fit selling like proverbial hotcakes. I could see Ninty with the proper marketing and the right software, a la a new Wii Fit type software that really keeps track of your progress, selling it in scads. What are your thoughts?