They're Both Great Guns!
I've got a G21 which I've converted to 10 mm.
A friend of mine has a First Gen G20, which he uses as a hunting back up.
The 10 mm cartridge is the most versatile auto loader cartridge going. In its common form, it's comparable to .40 S&W over a broad range of loadings.
But loaded to its potential, it's an excellent hunting cartridge, exceeding .40 S&W and .45 ACP cartridges, moving easily into the .357 magnum performance range.
Here's a rundown of some of the .357 magnum and 10 mm Double Tap ammos:
1) .357 Magnum, 1600 fps, 710 ft. lbs., 125 gr Gold Dot
2) 10 mm, 1600 fps, 767 ft. lbs., 135 gr Nosler
3) .357 Magnum, 1400 fps, 688 ft. lbs., 158 gr Gold Dot
4) 10mm, 1475 fps, 750 ft. lbs, 155 gr Gold Dot
5) .357 Magnum, 1200 fps, 640 ft. lbs., 200 gr Hardcast
6) 10mm, 1300 fps, 750 ft. lbs., 200 gr Hardcast
Then there's the whole sixteen/eleven shots thingy!
10 mm is, in its extremes, loud, harsh, flat shooting and controllable power!
That 135 gr Nosler has to be one bad boy of a SD cartridge!
It is the only thing I shoot that people routinely ask me, "What the hell are you shooting?"
I'm serious. It happens.
You expect such muzzle blast from a big caliber wheel gun, but not a black tactical pistol!
Get the right ammo!
The G30 recommended by Gunner is one sweet shooter--I've rented one twice! It is likewise convertible to 10 mm...
--Ray