I've got a G500, and a MX518.
MX518 is heavier. Thumb buttons are better, the thumb buttons on the G500 feel cheaper but do the job. Mice are very similar. The flywheel on the G500 is a gimmick, but works fine with the click. The texture on the G500 on the thumb rest is really rough for some reason (like some very fine sandpaper), and takes some time getting used to.
Overall, I prefer the G500 over the two, for the weight mostly, but the MX518 is a good mouse as well. It feels sturdier and less nonsense.