What if by 2010, Sony will debute the Slim version of the PS3 at $300; at the same time...Microsoft X360 Natal Sku will debute at the same price.

Who will have more sales, momentum going forward if this is the case (a very realistic scenario).

    vs   

The slimline PS3... but more importantly, you forget the PS3 remotes.

Well, a PS3 and the motion control remote / Itoy could be packaged together as well but will increase the price of the Sku so it will cost more than the Xbox Natal Sku from Microsoft.

lol a xbox360 with natal at 300$ hahahahahahahaha
natal alone will most likely be 150$ to 200$ microsoft lose enaugh money with the xbox they don't need to loose more with natal

There were only rumours of Natal costing $200 i doubt that its actually true though. I would say that the PS3 slim would win marginally. Like the same gap we have between them today, between 10,000-40,000.

I think the Natal Sku would win. I'm almost certain on this. However if sony decides to release a sku bundled with their own motion control it will be much closer.

More details are needed for your natal sku scenario. Is it the arcade sku? All skus? The slim ps3 is gonna have a 120gb HD. The type of people who'll flock to these units don't care much about memory but the larger memory can seduce some people. The core players however will choose the larger memory.

How sonys motion control performs is also a big factor.

The Wii....

PS3 slim is just a slimmed down PS3. Doesn't add any new functionality or value. Not going to sell a lot better than the PS3 fat.

Natalbox is a 360 with added gimmicky motion controls. It's not going to sell until they start developing good games for it. Third parties aren't going to release anything worthwhile until a year or so after it launches (As the wii has shown, third parties are timid in relations to gimmicky motion controls, and have a hard time developing for new control schemes). Unless Rare develops a game better than Wii Sports, Natal is never going to catch on in the casual arena.

The 360 of course.

I think it's too soon to tell for either system. Give it another few months, maybe closer to the holidays and ask again. You're likely to get a better and much more educated response.