Well you're both wrong, which one will sell more? - PSP, why? Not only will it have better games on IMO, but it has much better features i.e. mp5 player - music, video and games. The discs are UMD's (Universal Media Disc), meaning you will be able to use your PSP as a portable ZIP drive as you will be able to store files from your computer on there by transfering them from from your PC to your PSP via the UMD disc. Yes the DS has a cool little feature with the double screen, BUT I think I'd prefer focus to be put on the actual practicability of the console, which is what has happened with the PSP whereas the DS is slightly more centred around the idea of launching a slightly new way for games to work.
I will definitely be buying a PSP infact I have the money already saved up for the deluxe edition (?????????150, $250). Although as so many Japanese people have bought a PSP that it is outselling expectation by nearly double what Sony were expecting, which has meant the European launch of the PSP has had to be put back ..... till when I don't know.
But for me, the PSP is going to be far far better, being a fan of music and having enough rollercoaster videos on this computer to fill all of the space on a PSP's hard drive ... I'm sure the hard drive will be full straight away with music and videos for me to watch during free periods in school that are otherwise COMPLETELY boring