ok, one. The PSP has better looking games as well as a wider variety of them. IT plays PS1 games as well... and NO ONE can argue that the ps1 game selection is bad. In fact, the PS1 has the best line up of games ever in my opinion. Final fantasy tactics, FF7,FF8,FF9, Xenogears, wildarms...ect
The psp not only is open to play all PS1 games but psp games that I must say, are amazing. It can play movies (avi, mp4..ect, it actually has a media player), Audio (the PSP stereo system out does a DS one by far)(mix music, have digital displays and let the sony product communicate with your computer. THATS right, you can stream VIDEO AND MUSIC from your computer and have it play on your PSP.) You can play nes, snes, gba, neo geo, N64, Sega...ect all those favorite in one handy device.
Not to metion ebooks too.