The answer is simple.
Which sounds more appealing to you?
Playing
1) Animal Crossing
2) Advance Wars
3) Mario Kart
4) Mario 64
5) Another Code
6) Castlevania
7) Wario Ware
8) Meteos
9) Final Fantasy 3
10) Yoshi's Touch and Go
plus all the great GBA franchises like Fire Emblem, Metroid, and Golden Sun.
or (tryin not to name too many racers, the PSP has so many)
1) Gran Turismo 4
2) Mercury
3) Lumines
4) Grand Theft Auto
5) Ridge Racer
6) Tales of Eternia
7) Hot Shots Golf
8) Darkstalker's Chronicle
9) Metal Gear Acid
10) Wipeout Pure
plus the ability to play media (pictures, music and movies) from your own PC as well as UMD movies.
Each has wireless multiplayer and great potential for online play depending on how the companies choose to embrace such things.
PSP has awesome graphics. The DS has a touch screen. PSP plays media. DS plays GBA games.
The choice is yours.