I believe the 10mm is the Glock 20. It's exactly the same as the 21 dimension-wise, just chambered in 10mm. I rented and shot one when I still lived in North Carolina, and found the full throttle 10mm loads to be just a little too hot for my taste in a defensive handgun, while the .45ACP is still plenty powerful enough, and is far more controllable and pleasant to shoot.

Both are great guns though. One can't go wrong with a Glock, IMHO.

Mike