DS by a longshot. Own the DS, love it to bits. It's just an upgraded GBA at the end of the day but the touch screen is awesome, you have a ton of 'mini' games like Tetris, Brain Training etc that you can play for the 10mins you're on a bus, then you have full on games like Castlevania that you can just sit and play for hours. There's games for 3 year olds and games for 70 year olds. It's actually the best selling console in the UK because it is for anyone and everyone, plus it has built in wifi so you can play with people nearby using "download play" (single card for some games means only one of you has to own the game, others you both must own the game) or via a wifi hotspot all over the world. There are also plug-in type cartridges that let you browse the internet and listen to mp3s
PSP is shit. Sony has been losing on sales since they brought it out. Good points: it can play and store movies, music and pictures. Bad points: all the games are PS2 ports, the graphics are no better than a PS2 and it takes forever to load anything. Me and Mole watched a vid on YouTube once... it took 7 minutes from putting the UMD in the console until the person was able to get into a fight on Smackdown v Raw. 2 of the loadings were to load the bloody loading screen. It's aimed mainly at blokes 18+ really, though it has the typical disney film games too plus a couple of puzzle ones. Only game I've ever thought was actually kinda cool is Loco Roco, which feels far more like a Nintendo game than anything.
Sony rep was in a couple of weeks back; official word was PSP is selling badly (we noticed) and so they were putting the price UP because they needed the profit from it (could this be further proof the PS3 is flopping?) They are however attempting to fight back by bringing out PSP slim in September: around 19% slimmer and 33% lighter, available in a couple more colours and *rumoured* to have dtv playback. That could be bullshit though. I still think that will not sell well tbh, only to those who want an all round portable media centre.
Nintendo rep was in 2 days ago; they're expecting us to be short on DS' now until after christmas because they're still selling so well. Obviously it helps that its summer holidays but with More Brain Training just being released, there are a lot of adults buying them for themselves too, or upgrading (finally) from their GBAs. And of course once summer is over, the christmas rush will start.
We also get more PSPs back faulty than DS. Not *too* many, I have to say it seems a fairly sturdy machine (though they have a lot of problems with dead pixels, always have) but DS virtually never comes back, unless its one of those retards who thinks if they drop it down a well onto a sharp rock, then put it in the washing machine to get the crap off it and then wonders why it isn't working anymore...
About the only cool thing for a PSP is that with some ps3 stuff you can use it as part of the game (F1 it acts as a rear view mirror, woo) but the DS will also do that for the Wii (first game to do it will be Pokemon Battle Revolution)
And as said, for games generally PSP you can buy 'em all cheaper on the PS2 and they're the same games. Shooter, shooter, football, shooter, disney game, shooter, racing game, shooter... DS they're generally more fun and involving but not so graphically good (though its capable of some awesome stuff, take a look at Nintendogs screenies, for a handheld its amazing, I'd be impressed with those graphics on my Wii) and a larger range, shooters, puzzle, action, sport, educational, disney, racing
Ooh but both are available in pink

Coz we know you want a pink one.