Well...I'd say my stock Matrix...but that's because I've never seen or fired a gx3 cocker!
Sorry dude, I can't really help you on that one...but I do think that cockers are reallllly sexy, and you should always think about what you want in a gun: speed or feeling. Both guns have fairly good accuracy, but you'll have to put on a nice barrel to get that accurate. Anyway, I like my Matrix, and the best qualities in it are speed (!), accuracy, and price. You should check out for yourself what you think is the best gun.
Hope this helped,
Eddepet