The G-21 is basically a SA only pistol with no safety and no chance of a second strike in case of a weak hammer strike
The biggest one was the ease of maintenance. The Glock was just much easier to maintain at both the standard field strip level and at the complete detail clean.
I'm 5'8'', 160 lbs and carried one for about six months. It's no hot-weather gun, but it can be successfully carried by an average size person. Great gun, no malfs, low recoil. Sold only to fund another purchase. Sorry, no experience with SIG .45's.For CCW, the G-21 is out of it's league unless you are 9 foot tall.