The PsP is a tough case... Its nice because instead of only being a gaming machine, it's also a portable multimedia player as well. You can slap pictures, movies, and music on that bad boy. You can also buy UMD movies for the psp as well, but thats kinda sucky since they still cost as much as a dvd in most cases.
The games on the psp, however, are subpar. There are several good ones, don't get me wrong. Loco Roco, Lumines, Mercury, smart bomb are all good examples of great games on the psp. But sony is too busy trying to take successful games on the ps2 and just turn them into crappy rip offs for the psp. Nearly all of them aren't worth it. Even because theyre portable, unless you really use your handhelds in a situation where you cant plug in a ps2, a lot of the time, it probably isnt worth it.
The DS, however, have tons of original games. You already have one, so you know. Id say take a good look at the games on the psp and see if theyre are actually a good number of ones you want before deciding. Most of the people i know who's purchased one has been fairly dissapointed. Also, if you already have an mp3 player, the psp becomes even more pointless.
But if you've got 170$ and more for a decent memory card, and can justify spending it? Well, go ahead!