Deathadder: For palmers/clawers/versatile players. The best FPS players use mice with similar egonomics. I have a zowie EC-1, which has almost identical ergonomics, and I'm probably one of the best FPS gamers, not because I'm skilled or play a lot, but because my mouse is a perfect fit. Not too many features, no need for drivers; simple and beautiful. A mouse is a tool that should immediately give you power in games; you paid money for it and it has a simple purpose. It should do exactly what you need it for, no compromises.
G500: I bought mine new for $35 at Best Buy, and I don't even feel it was worth that much. Great tracking, but the ergonomics are atrocious; I'm 6'5" and have larger hands, but this mouse destroys my hand. I get sore quickly and everything takes too much effort. Takes a lot of getting used to. Adjustable weights are useless for me, too many buttons, the scroll wheel is THE WORST you can buy, since it takes surgical precision to hit middle mouse, and it doesn't feel as smooth as it could on my mousepad. Sides have an awkward synthetic rubber feel and actually make my hands chapped; the grip will start peeling and getting gross even if you're as clean as I am. Uncomfortable and unprofessional.
Verdict: Deathadder, by a damn mile
. Did I mention it's optical?