I got a PSP not too long ago, and i'm enjoying it very much. I've used my friends' DS' before, and they really don't compare to the PSP. Yes, the battery life is a lot shorter, but that's because it's supplying a huge bright ass screen, and some pretty nice graphics. If you need to play more than 6 hours straight, then just get another battery, they're not expensive.
Keep in mind, that if you mess with the firmware like some of these posts are mentioning, you need the PSP 2000 version. If you go with the newest, the 3001 (also called PSP "bright") then you won't be able to use all the custom stuff out there. They haven't cracked it, yet.
Anyway, I don't think you'd be sorry with a PSP. They have good RPG's, have a couple fighting games (with more on the way) and a bunch of other stuff. I don't regret purchasing mine at all.