If you are left handed, then the answer should be obvious.
If you like to grip the mouse primarily with just your finger tips, often flicking the mouse - Copperhead.
If you like to grip the mouse with your entire hand and palm and make fast, long, sweeping movements with lower sensitivity settings - MX518.
Otherwise there really isn't that big of a difference to be honest. People will swear by one mouse or another, but most people only own one mouse, and they're just doing the typical fanboy thing in trying to further justify their decision.
Other factors to consider:
Copperhead has onboard memory that stores customized profiles, allowing you to setup the mouse to work the way you want it to, and then it will be able to function that way without drivers no matter the computer you plug it into (including if you setup macros to the buttons, increase the polling rate, etc...).
MX518 has extra buttons in a more functional position and has a sensor better suited to very low sensitivity play.