I think the NGP is pretty dang cool, I love that it has a OLED screen and 3G wirelesss (especially the latter). But I do have some issues with it and I think these issues may become incrediably problematic:
1) Battery life. I believe Sony stated it should have the same battery life as the 3DS roughly, 3 - 5 hours. I would imagine this can only be achieved by using a higher quality battery which can significantly raise the price of the unit, and the trade off is not exactly what I call impressive.
2) Price, both hardware and software. The technology in the NGP is cutting-edge, there is going to be a significant trade-off somewhere and its going to be price. I don't see this launching at $300, $400 at min. The 3DS is also cutting edge (in a different notion) and its asking price is $250. OLED, battery, touch panel, GPU, etc etc is all adding up to make it very expensive. And while Sony may be willing to take a significant loss with hardware sales, I don't see that changing the general price structure. Its going to be damn expensive.
There is a potential possiablity that by accepting a contract with a 3G data carrier the price of the NGP may recieve some form of a discount, but I'm not sure if the 3G infastructure is like that of a cell phone or more like Amazon's Kindle and its Whispernet service. So we'll see on that end.
And what about the games? 3DS titles are retailing anywhere from $35 - $50. There's no doubt NGP will hit somewhere between $50 - $60 (and potentially $40 but extremely doubtful).
And while the NGP may have a better launch lineup, it comes during the Fall and by that time, the 3DS will have built up a significant lineup of software and I'm more than certain Nintendo and its partners will have a full lineup of new software at the time to put a "mute" on the NGP's launch. Further, supplies should be replinshed at that point, only pushing sales further.
It'll be damn interesting, that's for sure. I'm likely to buy both but that's because I

ing can.

But if I had to pick a potential long-run winner, my bets are with Nintendo.