I seem to be in a sine-like periodic favoritism between my DS and PSP. On the one hand, my DS has great games and very innovative touch screen functions, as well as some great online titles that people actually play (Bleach, Advance Wars, etc.). Also, there's the first-party Nintendo games that almost always are a lot of fun.
The PSP, on the other hand, has several emulators that run at 99.9%, as well as having a few of my favorite music albums on it. Plus, Disgaea is one of my all-time favorites, and having that ready to go at any time is something I would have killed for back in 2004 when I first got it. And of course there's Crisis Core.
I have to say though that it's a good thing that the custom firmwares for the PSP came along. I was extremely close to selling my PSP in early 2007 because there simply wasn't any great games to play (that I hadn't already bought and beaten). After dropping some NES and GB goodness on the PSP, and seeing how the "scene" was producing some top-notch stuff, I held on to it, and I'm very glad I did.
Anyway, I'm currently on the DS-lovin' due to The World Ends With You and Final Fantasy III. After that, I'll be checking out R-Type Command for the PSP.