I have a DS Lite, comparing it to the games available for the PSP, it's got all the old-school Final Fantasy games. I used to emulate them on my Pocket PC, but they never played back right, and things just didn't work well. The DS Lite is so much nicer, even if the games are a little updated and different. (Final Fantasy I has completely new graphics, music, etc, so no 8-bit graphics and music)
Also, the DS Lite/DS Classic browser just came out (Opera fans rejoyce) so the DS now has web-browsing capability. Unfortunately, to use it, you'll have to plug in both the DS card and GBA card slots with the Browser and Memory pack. I have no idea if it works well, but if it works near as good as the Opera browser for Pocket PC works (minus downloads), it'll be a worthy purchase.
Though, the PSP has the GTA games.. that'd be a reason for me to pick it up. But, I absolutely love the old-school Final Fantasy series, and it's totally available for the GBA/DS, and has been for a while whereas Final Fantasy I and II are barely being released for the PSP.