The MX518 is a 1600dpi mouse with a 500hz polling rate. The G5 & G7 are 2000dpi mice with a 500hz polling rate. The razer diamondback is a 1600dpi mouse with a 500hz polling rate that can be hacked to 1000hz (like mine), and the copperhead is a 2000dpi mouse with a 1000hz polling rate. It won't make much difference, but technically speaking, the razer mice are more accurate because of the polling rate (response time). You probably won't notice the difference, but it's worth mentioning. Some people don't like the shape of razer mice, so they get logitech. Whatever you get, you can't go wrong with. The only one I don't recommend is the G7.
so, umm, err yeah