I have the 3DS winning this war ((just as nintendo always has))
My reasons for this are as follows. Nintendo has 2 of the top 10 gaming franchises under their belt, and to make it worse they are the top two, and they are purely exclusive. No one, i repeat no one can fight that. Yes Pokemon B/W is releasing for the DS but I know a lot of people are going to buy a 3DS and play them on that instead of the DS ((myself included)) Then there are people who will go out and buy a 3DS for the simple fact that they know Pokemon is coming out eventually and will be totally awesome. Mario is a great fix for Hardcore and casual gamers ((i don't know how nintendo does it, but they can make both parties happy with this game, go figure))
Second reason: the 3DS offers something new ((not saying 3d is new, but without glasses it is)) It's following that old montra of let's give them something to talk about. The sheer fact that you can play games and what the Heck they pop out at you is freaking great.
But this is not to say that the NGP will not sell well ((i plan on getting one... when i got some free cash and i see a game i need in my life like KH)) Both systems have a great support from 3rd Parties ((I wonder why people keep forgetting that the 3ds might get some ports from the PS2 as well, seeing as how they already have Metal Gear Snake Eater))
I do see how the 3DS can fail, i do see a lot of silly parents not understanding the difference between the systems and denying their child the newness on the market, or silly sales people not properly explaining to the parents that it's a new system ((nintendo really should call it something else... but then look at the GB, GBC, then the GBA... all same name and all sold great))
All in all i have to say that the 3DS will win mainly because of exclusives ((and when i say exclusives i mean exclusives)) and a experience you can't get anywhere else ((no offense i can get great looking games on my TV, but i can't get glasses less 3D... yet))
... Why did i use so many (())... as if I wasn't typing but actually talking and those are my inside thoughts... hahaha I'm crazy lol