First off, get a mouse which feels natural in your hand. Some reccomend the NAOS 3200, but i hate the shape. For me, the razer deathadder is brilliant. I have spent 6+ hour sessions with it and had no discomfort.
Razer deathadder, its used by many of the professional fps gamers out there. There's no need to get the 2013 version, the original is amazing.http://www.amazon.com/Razer-Deathadder-Infrared-Gaming-RZ01-00151400-R3/dp/B002Q4U5DK
The steelseries sensei is also a good mouse, and is my second choice after the deathadder.http://www.amazon.com/SteelSeries-Sensei-Laser-Gaming-Mouse/dp/B005L38QLQ
I had a Razer Mamba, and loved it. It was a great mouse, and if you're willing to drop that amount of money (I won mine) then go for it, but for me its not worth the extra cash over the deathadder.
I would tend to steer clear of the G600, or any logitech mouse for that matter, as logitech's customer service department are appalling. It took me 3 weeks to get an new headset when my G35 had a fault.
One of the most important things you can do is get a mouse that either does not have acceleration, or one on which the acceleration can be turned off, especially if you are playing FPS games.