hey there. I own a psp and and a DS and I am somewhat of a sony fanboy...psp has the latest firmware (cw) homebrew isos etc etc but honestly since i've got my DS, my psp has been collecting dust for one simple reason....I enjoy playing games, period.
Yeah psp has good homebrew, multimedia features, blah blah...but of recent, the games are rather sucky and still expensive. somebody mentioned that in order to get something out of the DS you need to buy a Flash cart...theyre right but in order to get anything out of the psp, you definately have to fork out for a decent sized memory card so in my opinion, that point is moot.
In terms of isos vs roms; roms win hands down. the graphic to file size ratio in the psp is really bad. you can have a decent library of ds games in a day, and with our country broadband situation, thats a real plus.
but for me the real winner with the ds is the innovation in gameplay. yes there are stinkers, but the gems really shine...blindingly!!
lets put it this way, if you want a little of everything in terms of entertainment (movies,music,internet,gaming) a homebrewed psp is probably the way to go, but bear in mind, it is homebrew and homebrew is...well...homebrew

sometimes buggy, requiring some sort of homebrew knowledge and no official sony (proffesional type) support....what I'm saying is that the psp just that, a portable playstation, not an amped up ipod. eventually you will find, like me, its sheer bulkiness will overshadow its media homebrew "coolness"
The DS has great games and gameplay and its repertoire of games is just growing...fullstop. Yeah, its also true that some game producers/coders use the touchscreen pathetically, but the its use is improving drastically (the world ends with you; drone tactics, etc.) and i see it only getting better.
Look, respect to the psp, its a great handheld; sony wanted to shrink the playstation and they did a bang up job but the ds is simply more fun.
so in the end its great fun vs arguably mediocre functionality...you decide.
hope this helps.