The Wii exists...runs away.
Seriously though, to decide how well either one will do, you have to bring the Wii into the equation. I mean this isn't the beginning of this gen, it's over 3 years in (4 if you want to go by the launch of the 360). Personally, I feel that it's a little to late to bring in casuals who have not already bought the 360 or PS3. They may bring in a few of them, but the majority aren't going to pay another $300-$400 for a new system and motion controller/camera that doesn't come with it initially.
For the most part, these controllers are going to sell to those who already have the system. This is a reason I think the Arc will fair slightly better (especially if you can play pre-existing games with them). There are quite a few people who already have the PS Eye, I'm one of them. So all I would have to buy is the stand-alone Arc wands, which will probably be ~$50 or $60. People could either get a used PS Eye cheap or buy the bundle, which I can see Sony bundling the wands with the PS Eye and a game for say ~$80-$100. However, I doubt that most of the hardcore shooter fans on the 360 are going to get into Natal, especially when most of the games are going to be going for the casual audience. Even more important is the fact with how much MS is pushing this as new tech, and with R&D added in, I don't see them selling it for much less than $100. It might even be ~$150. Also remember Patcher's original reasoning for why he said it would be ~$50, because "it's just a camera." I doubt he even knows what kind of tech is going in this thing.











