As a handheld collector, I have nearly all the handhelds made so far, and will definatly own the coming ones (Zodiac and PSP). I will probably pick up the N-Gage second hand, as it's price is outrageous, and I don't want to sign up for a phone contract, just for a games machine.
Anyone who says the Zodiac is no more than just a PDA is wrong. It features a video processor. Which allows programmers to draw to the screen faster than any other PDA that I know of. This will be the key when it comes to emulation on the system. If people make use of it properly, using assembler calls, most likely Snes, Genesis and Amiga emulation will be perfect as nearly the full CPU will be free for emulation purposes (as opposed to spending half a frame just filling the screen). Also, it has a great button layout with additional paddles so SNES emulators won't have to workaround the missing buttons.
The GP32 FLU is definatly the favourite of any of my handhelds. The screen is great outdoors. The frontlight is a lifesaver for indoors, but I'm not a fan of frontlights as they create a very washed out image. I am hoping for the PSP to have somethign similar to the TFT panel on my Cassiopeia PDA (really fantastic screen). I'm not even really that much of a retro gamer. I was really hoping for Amiga emulation, but the Atari ST has most nearly everything I could have wanted anyway (Stunt Car Racer anyone!). I'm not totally fussed about the SNES, and I alredy have a Nomad, so Genesis emulation isn't a concern to me.
Certainly, if you can afford it, get both. If you really are tight for cash, you might want to keep saving and get the best you can afford once the Zodiac scene is more mature. But if you are looking for instant portable retro gaming on a tight budget, there is only one choice. GP32 is where it's at.