hmm it all depends.
Ive owned 1 of the origianl DS's , then sold it cause i go bored of it , but thats just me.
However , i see the DS more of a Family orientated device , such as very young age group right up until when your too old to be playing or shouldnt be lol.
PSP is aimed mostly at the Teenager/Young Adults, with alot of violence and similar stuff, personally not my cup of tea. But PSP has a few adavantages of DS because of its Internet WIFI capabilties and MP3 playback, not to mention UMDS & MP4 videos.
So if your after features grab the PSP, or if its value DS.