I think there are a few too many unknowns to really make an accurate prediction at this point in time ...
First off, the NGP is far enough away from launching that it is entirely possible it could see a much later release than most people are expecting today; and if the NGP launched 6 months after the 3DS it would (obviously) have a much smaller sales gap between the 3DS than if it launches 18 months after the 3DS.
Then there is the price of the NGP and its games because it is possible that the NGP will launch for $400 (or more) with $60 (or higher) games. While people may see value in the NGP if it is cost competitive, the higher the price is the more likely it is that the NGP will struggle to find a market.
On top of that we have questions about third party support. To everyone who is paying attention the 3DS is getting dramatically better third party support starting at launch, and we know little about how third party publishers are supporting the NGP. Obviously, having stronger third party support can only benefit a platform.
When you start adding these things together and you ask how well the following systems would compete against the 3DS, odds are pretty good that you would have dramatically different answers.
1) A $400/$450 NGP with $60/$70 games launching 18 months after the 3DS with weaker third party support
2) A $300/$350 NGP with $50/$60 games launching 6 months after the 3DS with (roughly) equal third party support
