I own both the DS and PSP, so I'll go ahead and give my two cents:
Seriously, get the DS. Nintendo's been considered one of the best game developers for quite some time this year, and they're gonna keep releasing stuff for the Wii and DS. Sure, the PSP has a massive screen, and believe me, it's fantastic for media capabilities such as music playback, and playing videos at a really sweet definition.
But games-wise, the DS most certainly tramples the PSP.
And about the D/P question, be sure to look around at the D/P forum, since I'm sure we've had a ton of those threads already.