If you're unsure, I wouldn't recomend making any decisions just yet. The 3DS is still a month away (unless you're in Japan) and the NGP at the earliest will be out at the end of this year.

Personally, I know I will play the 3DS, and that I won't play the NGP, so I pre-ordered a 3DS a month ago when the price and date was announced. Speaking of price, based on past experience, the NGP is most likely going to be pretty expensive. At $250 the 3DS isn't exactly cheap either, but the NGP is supposed to be a handheld PS3. Sony is either going to take a huge hit on every one sold, or they're going to be expensive (or both, like the launch of the PS3).

The graphics of the NGP will almost certainly be better than the 3DS's, there's not really any doubt about that. On a portable, however, graphics are less important than if you're playing on a TV. They also obviously won't be 3D, which may or may not be important to some people. I don't care about the 3D that much; the games and other features have me sold. It's definetly a cool feature, but not what I'm buying it for. Plus, the battery on my original DS is going, so I will need a new DS soon anyway.

I believe Nintendo is doing mall tours with the 3DS right now, you could check to see if there's going to be one close to you and then you could go try it out for yourself. Or, after it's released, I'm sure there will be demo units set up in stores that you could mess around with.