500 = better shape/more useful buttons. i actually use the buttons below and above the wheel all the time, same for the side buttons. i use the wheel buttons for end/home for quick webpage navigation, thumbs for copy/paste. 310 i bought on sale and returned, shape is bad. only one side button works,s ince other is for lefties really. the app switch button on either is rather useless so it doesnt count. i set it to f5

the 310 is not a 510 sensor, the name is misleading. after looking at the 500/700/900, the 310 looks like cheap plastic junk to boot. the 500+ are sleek and don't have gaps in the case. basic mouse feel in the hand type thing