Santa brought a DS for my DS9 last year (when she was 8). She LOVES it (and so do I). Like everyone says it's very durable. My daughter takes it with her almost everywhere. And it plays GBA games too, so you can find cheap games everywhere (although the graphics are better on the DS games). I find GBA and DS games on sale a lot. Plus they come out with new titles and drop the price on the old ones (I got last year's Hannah Montana DS game for $9.99 at TRU a couple weeks ago after they released the new one which is $29.99.) I also like to play her DS a lot because games like Brain Age and Uno and some other board games are fun to me. Even her grandma likes to play Brain Age on it, and my mother is NOT a gamer!
My 15yo nephew has a PSP which he loves, but when we visited last summer he played with my DD's DS a lot. He said he didn't have a lot of software b/c PSP titles are expensive. But he also uses the PSP to listen to music and videos he downloads off the net. I can't account for it's durability b/c my nephew is 15.
So I guess it all depends on what he's interested in. If he likes graphics and wants to be able to watch movies, go with the PSP. If he just wants to be able to play a lot of games (and you want to enjoy it too) go with the DS.
I know this isn't much help, but it's all I've got. LOL