<a href=showthread.php?s=&postid=6869639#post6869639 target=_blank>Originally posted</a> by TheFlyingTang
Ball valves are prefered because they will control water flow, and when fully open the diameter of the opening is the inside diameter of the pipe. In a gate valve the opening is much smaller and restricts water flow- something you don't want to do in an aquarium set-up. There are enough restrictions with height and amount of tubing bends that the water has to make in order to return to the tank.
If you feel that your ball valves are getting "sticky" then just exercise them once in a while.
Hope this helps.