As I said the MX 518 is a great mouse. I have it.. ANd haven't had a single problem with it.. I was deciding between it and the Razer and went with the 518. It's newer, had 3 sensitivity buttons (if u ask "why do u need it", my answer is "why not. Same price!"), and looks more presentable..
It fits my hand perfectly.. It's only right handed btw.. Not sure how the Razer is.