Since you had a GBA, I assume you still have some GBA games? If so, I recommend DS Lite, which will allow you to play GBA games you get and probably have more franchise you want. I would recommend PSP if you are interested in homebrew, or media functionality for your handheld. Since you like games on both handhelds, I won't mention it beyond that. Also, DS Lite is cheaper, but I would recommend trying for a special deal when one becomes available (same for PSP, if you decide to go with that) There were quite some cheap DS Lites near christmas at the end of last year.
I am a PC gamer, and also have a NDS now, but without access to a Nintendo Wii until End of 2007.
Currently playing: Super Smash Brothers Brawl(Wii), Mystery Dungeon: Shiren the Wanderer(DS), Dragon Quest Heroes: Rocket Slime (DS), WiiFit(Wii)
Games Recently Beaten: Final Fantasy Crystal Chronicles: My Life as a King (Normal; Very Hard after the next DLCs become available)
1 word: RTFA