The Wiz is a fantastic machine with a beautiful, beautiful screen. It doesn't really do much better than the GP2X software-wise and the screen is slightly smaller. The battery (built in) is much better as are the controls. However, it has one pretty big problem (depending on software) and that's screen tearing. Some games when scrolling or if there's fast movements tear the screen diagonally - it's quite noticeable at times, but it's something you get used to. To be fair, most of the newer software did not have this problem as devs overcame it, but it does still plague some games and apps. However the fix also takes CPU time away from the software, potentially slowing it down.
The Cannoo is another really nice machine (you didn't mention it above) and I actually prefer it (comfort-wise) to the Wiz and the GP2X machines. But it lacks the Wiz's excellent screen, but like the Wiz does have a built in battery, which is a big plus over the GP2X. The Cannoo is everything the GP2X F00 should have been. It also features a nice analogue stick.
You would be pretty lucky if you can get a Wiz nowadays - they are pretty rare beasts, however you are much more likely to get a Caanoo.
As for GCW it looks like an excellent machine, but it's lacking software at the moment and several emulators. I don't have one so can't comment on software slowdown etc.