Psp screen is bright and exceptionally clear. Better controls and graphics. The Memory Stick Duo makes it easy to trade save files by swapping cards. But when it comes down to battery life, bring back up.
Seeing that my DS' library is 3-4 times my psp's, I think it's safe to say that I like my DS better, sucks that many companies pulled their support away from psp too early.
PSP games are still going strong in Japan but not many company dares to localize them, shame.
That's an interesting way to compare them, and works pretty well for myself. I feel about the same towards each system; they each have strengths and weaknesses. Looking at my library, I have about 10 more DS games but I also had the system longer (don't remember exactly, but at least a year). So it's roughly the same number of games on each. The PSP definitely has better graphics, which is its real strength.