Hoser - for durability and speed in a training holster, it's hard to beat Kydex. I have a Comp-Tac Settable Cant that works with either belt loops or a paddle (I've never used the paddle - only the loops over my Wilderness belt).
As a matter of training philosophy, I've had something of a conversion, though, and now only train using one of the two holsters I routinely carry concealed with: an Alessi PCH for my Commander or Concept X, and a Sparks VMII for my Custom Carry or S70 Government Model custom. The VMII will be replaced as the five inch gun carry holster with another PCH the day it arrives in the box (with an entire rig of belt and mag/light pouch) from Lou.
I seldom carry OWB, but when I do, it's in a CQC/s. Theoretically, I would think that an OWB with slots matched to your gunbelt would be slightly more stable over hundreds of presentations than a CQC/S, but I like the CQC/S so much that I wouldn't hesitate to use it for a class. I think it's plenty stable enough for that kind of use.