I was considering the same 2 mice. I just picked up the G303 and it is way too small for my hand and will be returning it. I have been told by others that the G Pro is the same general shape as the G303, so if that is comfortable for you then it may be the better option. The G403 looks a little bigger and is what I am leaning towards. You might want to try one out in a store first, that is if you have a PC shop nearby.
9/26/16 at 12:24pm