as san mentioned, there is a G5 with two side buttons ( which addresses the main weakness that the previous G5 had, a really bad decision move by logitech, but many people with fat thumbs complained they would hit the second side button by accident with the MX518 ). However the MX-518 and G5 feel significantly different, the G5 uses an anti-grease rubber material that feels like a bottom of a rubber boot. I for one don't really like it compared to the smooth plastic feel of the MX-518, my advice is to try both out at a store and take whichone u think feels better.