well, psp are more to high end handheld graphics, well... most of the game, how to say this, only be easily played if the player played game alot.
an inexperience player would have lots of problem asking how to play this and that. well, it's just my opinion as it happen alot to my friends who don't play games alot.

while for the ds, it's kinda friendly kinda of game, u do see hardcore game on DS but seldom u do see 1 coz nintendo are mostly off to more friendly and party kinda game. and it's easy to get used to the new style of gaming.

either, it's ur choice but i would prefer u to get both of em if you can afford though. I prefer something light and quick during im playing game outside. Something like i can just switch on and off w/o worrying about the save as some games on ds are just "play for fun".

both system is targeting different ppl.