I say buy both if you can afford it!!!
Personally I go through stages. One period, i'll be totally into my DS and then another period (like right now), i'll be into my PSP. Right now, i'm addicted to Tekken: Dark Resurrection for PSP. Might as well get both so you never get any feeling of regret for choosing one and not the other. Bought both and am quite pleased at that choice.
If you like emulators and such, then get the PSP. I used to do that but upgraded firmwares to play legit PSP games (i.e. Tekken). But that's ok cuz my bro has a PSP with all the emulators and such.
GET BOTH!!!!