Yeah, I own both
I don't see much use in my DS though. I don't really enjoy any games on it, and I prefer my games in a more crisp 3d which the DS just isn't powerful enough to support. (Some games are 3d, but the graphics are just horrible in most cases)
The only advantage to the PSP is internet. It barely even works, though.
Barely works? I use it all the time and it's actually pretty awsome. Do you even have a PSP?
The PSN store is also quite awsome on the PSP. If you need something new you can download it anywhere there is internet. Plus it can be used as a video player or music player and the speakers are fairly powerful for it's size.
Obviously I think you should get a PSP, but it's up to you and your budget. :P