Quote Originally Posted by HDNitehawk View Post
According to the 5D III manual you can assign a button in "custom controls" to switch between AF One Shot and AI Servo mode.
A great feature. The 1D X has two M.Fn buttons, one of which is next to the DoF preview button and I can easily reach it with my ring finger (in landscape orientation, that is - I'm actually hitting the lower one on the portrait grip). I've got that button assigned to switch between One Shot and AI Servo, and it works very well.

There is a downside to that, though, but it applies only to the 1D X and not the 5DIII. The 1D X uses the 100K pixel metering sensor for face tracking during phase detect AF (Canon calls that iTR), but that requires auto AF point selection (61 pts) or Zone AF. If I'm in Servo mode using iTR and press the M.Fn2 button to switch to One Shot, I'm still in 61-point or Zone AF, when what I want is to be in single manual AF point selection mode.