It's a tough call. I kept my DS for about four months, the lack of good games really stung. It's a shame that a good chunk of the DS' library is made up of GBA ports with touch-screen minigames...or N64 ports that aren't even optimized for the system. If you're into Nintendo, just go ahead and go with the GBA SP.
The PSP is a very nice system and outside of games it has some very nice features if you can afford the extra memory. It's already replaced my iPod and I REALLY like the video features. Not to mention that the launch line-up was ALOT better than what was found on the DS. However there is a disturbing amount of racing games being released... You're also getting a complete console experience with the games--I'm highly anticipating the portable Devil May Cry. Also, given the fact that the thing connects to a computer via USB, it's only a matter of time before someone figures out how to emulate 16 bit games and even GBA games on the system.
Now, I'm a console whore. If a new console or AAA title comes out, I buy it. Since you're more of a casual gamer, wait until E3. Wait until you see some of the heavy hitters in action before you plunk down your $150-200. (I say $200 because the non-value pack PSP will probably be about that.) In my opinion, the DS is really fucking up. Nintendo isn't porting the games it should be and games that are pretty much done won't be out until fall because there are fucktards who want handheld games with online capabilites. It would be beyond easy for them to port something as simple as Super Smash Bros. and have it be a hit...
