Definitely the DSi. Even though the specs of the PSP might look better on paper, the important thing is whether your son would like the games available on that system. If he wants the DSi he's probably be disappointed to receive the PSP instead, even if the PSP was better in every way. I'd say the same thing about someone getting an XBox 360 or PS3 when they want a Wii - the specs aren't the important thing; the games are. If someone wants a particular game system they will almost always be disappointed to get a different one, even if it seems better to the person who is buying it.
Having said that, I agree with the previous posters that the PSP isn't as good as the DSi. Even the older gamers I know are not particularly impressed with the PSP. It just does not have a great selection of games at this point. Everyone I know who has both uses their DS or DSi much more than their PSP.
ETA - DSi is newer, but both are still being sold. The DS games are playable on the DSi. There are some downloadable games that only work on the DSi and not the DS. (Mostly the ones that use the camera, which the DS does not have.) The DS can play Gameboy Advance games, and the DSi can not. So it's a tradeoff - do you think your son would enjoy the camera or the GBA games more?