Well, I would suggest the DS, if you wanted to buy a handheld...
its cheaper than the PSP, and the only thing I've seen thus far that it doesn't have is the propriatary movies and the slightly better graphics.
All the other features, people seem to get mixed up... the DS can do practically everything the PSP can, although those features may not be in use right now, they will in the near future. They've recently announced that the DS can play over the internet via your home/any wireless network. And it can play movies, if you buy a GBA movie player (stereo DS sound) ^_^ it can also play MP3s with a simmilar device, (maybe the same one, havent tryed yet.) and so much more. XD
In the end, the DS will probably have more games out for it, and if not, it can still play GBA games, so you don't have to buy a GBA for those. ^_^;
Rather than do another side by side comparision, I just suggest you get a DS. XD