The G5 and The Razer Copperhead are almost identical. In fact, they use the same optics. The difference lies in the firmware and drivers...and design of course!
You can't really make a good decision until you've felt the mouse in your hand. I really liked the Copperhead that I had with my last setup, but I was experiencing some hickups with the drivers. Could be my rig, so I can't really say if the firmware or drivers at Razer needs improvement.
Pick the one that feels best, cuz they're both the same. Literally.