LOL rcx! Not much I can say that these guys haven't, but I know it's good to get as many opinions as you can, so let's have a stab at it. We won't even consider the N-joke; this race is clearly between Gamepark and Tapwave.
Zodiac advantages:
Newer technology and better specs.
Cool, slick and slim black design with aluminum casing.
The website and documentation is in English.
PDA functions if you like 'em.
Hundreds (?) of good simple PDA games, homebrew and the like.
Six buttons.
PDA emulators, weak though they are right now.
http://www.zophar.net/ppc/palmos.phtml
Full support from Prophet who, truth be told, usually knows better than you and me.
http://www.zodiacgamer.com
GP32 advantages:
Established "scene".
Less expensive.
Four buttons. Any purists out there?
Always working, if too sensitive, control pad.
HUGE number of playable emulators and games.
Competitive homebrew scene.
Supporting a company that may actually like fun more than money.
Probably more comfortable than a Zodiac.
Definitely has a flashable bios.
To me, the GP32 comes out on top, but really only time will tell. I haven't seen a battery and backlight comparison, if that's important to you, but it would be interesting. From what I remember, several dedicated and talented authors like rlyeh are putting time into emulators for both handhelds, so I am afraid to make judgement on which one has or will have more support.
I'm staying away from reviewing commercial aspects, as I don't care about the abilities from either system in these regards. New handheld console games are typically overpriced and have better counterparts on larger systems. The best console games are the ones that either look or feel better on a handheld -- games like Tetris, Day of the Tentacle (the pixellation has disappeared!), Pokemon, and countless arcade classics like Asteroids, Qbert, and Space Invaders. I can't imagine Tony Hawk will be more fun or beautiful on the Zodiac, as ports of 3D games (Tomb Raider on N-gage) have so far been watered-down versions of the real thing. Authors need simply to take advantage of handheld abilities, to not view them as crippled Playstations.
Six buttons being better than four is debatable. The best games ever made, with few exceptions such as Zelda: A Link to the Past and Street Fighter 2, require four or fewer buttons. And what doesn't help a console only hurts it. Four buttons opposite the control pad on a Zodiac may be useful as it's a PDA, but the lesson here is buy what you need.
IMHO, add a grain of salt, and all that jazz.