Personally I haven't had too many issues running GBA on native a320, though my dpad is a bit shit and the L/R buttons are awful. The GCW looks superb, and I think it's pretty close in price to the a380 (are they still running around $109? Zero is $120 for the standard model, though you'll be paying more for shipping if you're outside the US). The Zero also has the advantage of 4:3 aspect ratio so you can run it full screen without horizontal stretching. This is also purely speculatory, but given the public response to the device you're likely to have much more community support with the Zero. We haven't seen a great deal of GBA emulation on the Zero, but what's been shown looks pretty flawless. Then again, I don't think we've seen titles like Mario Tennis so the jury's still out there.
If you don't mind waiting a couple of weeks til the Zero hits consumers you'll have a much clearer answer then.