Well, they're the same price, so flip a coin.
No, I kid.

I'd buy the PS3. I have both, and love them, but I think there's more substance to the PS3. I mostly have my PSP for internet browsing in bed, and collecting classics that are re-released on it. Things like Street Fighter Alpha 3, Powerstone Collection, Capcom Classics, Metal Slug Anthology, Castlevania, etc. There's also really great PSP exclusives like Guilty Gear Judgement, Brave Story, and of course God of War.
That said, my PS3 is something I play pretty regularly. I love the store and downloading new demos and stuff all the time. Its like having a 360 that exists in some alternate universe.

Then the occassional multi-platform game comes out and you'll get it for PS3 instead. (Soul Calibur 4 for example, then again, multiplayer games are typically better on 360) If I were you, I'd try to get a 60GB for backward compatibility. They're totally money.