I tried homebrew for the PSP, its easy and fun. I imagine it'd be harder for the DS (Keep that in mind if you try out homebrew).
The PSP slim also has larger memory, but is also (from my understanding) harder to get homebrew to work on. If you aren't interested in homebrew get the PSP slim. I *had* both a DS and a PSP, but I sold them a while ago to get the sum of $120~ (separate times though, so each time I had only about $60~).
My DS was fun, but my PSP was better for me. It was funner, and it had the games I liked, not pokemon (The new super mario bros sure was awsome though). I'd personally rather have a PSP than a DS, but 3 years ago I would've prefered a DS to a PSP mainly because 3 years ago I was much more into mario games.
Hope any of that senseless rambling helped.
Cheers,
-naota
I used to do a lot of time traveling when I was younger. I called it tequila. I would drink this potion and wake up 3 days in the future! 'Where am I trusty dumpster? Get me home!' - Craig Ferguson