To be honest i'm getting a bit bored of the PSP bashing, every time theres a thread like this it comes along.
Personally i have both, and whilst i play the DS more i hardly ever play it on the move, particularly games like nintendogs and phoenix wright, which you need to yell at the screen, and trauma centre where a shaky train doesn't help.
Hence my PSP gets a lot of game time with everybody's golf, virtua tennis, twisted metal and locoroco.
Its not a bad machine by any accounts, although i would say only buy it if there are some games you particularly want, not having a PS2 made it good for me as i'd never played their older brethren.
Hopefully sony will release their downloadable games thing they've been harping on about and get a decent version of FF7 released, then the machine will have a new lease of life.