1. If you're a hardcore gamer, such as myself, than I'd get the PSP for the games. The PSP has more detailed storylines and the games are meant for an older age group. The DS system is for like people who like to play a couple hours a day and then quit. They have those games such as Mario or Racing type of games.
2. The best system between the two for non-serious, chilling-gaming is the DS system. You can sit back, relax and enjoy your DS with photos, music or the internet. The PSP you can get sucked in for hours at ease lol. I sure do get sucked right in on that system

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3. The PSP is definately a winner in this category of questions. The bigger screen allows it to show more. The processor on the device is bigger and better allowing to load it faster with better quality. And it also allows to have multiple tabs up and running at once. Versus, the DS system where it does have a browser, yes. But it can't watch vidoes on it from YouTube or anything. The PSP can. So I'd choose the PSP on this one.
4. I think you would have a little keypad on the screen and scroll over the buttons on the screen with like one of the buttons on your PSP I think. I'm not very, truly positive on this one lololol!
In all, the DS and the PSP level out depending on the circumstances.
I hope this enhances on your choice!
-Kyle