Looking for some buying advice to help me chose between the Canon BG-2EN grip or the Opteka battery grip. First and most important question. I'm aware that these grips don't have an Autofocus on/off button but do they autofocus on a half-press of the shutter release button on the grip? Does either of them have a main dial to replicate the functionality on the grip? Is the Canon version significantly different to justify the higher price? The grip will be used on a Canon 50D if that makes a difference. Thanks.