I dont have a G21, but I have a G29 (10mm compact - G30 is the .45 twin), a G19 (9mm) and twin G23 (40s&w), as well as several 1911's along they way and an XD40. Hands down the glock is my favorite choice. The 21 is pretty large (not too different than the 1911), but the G30 (doublestack)or G36 (singlestack) are both smaller versions if you are looking to conceal carry. I chose the G29 over the G20 (twin to the G30 / G21) because the G20 was a little too big for conceal carry in south FL. Ive shot both the G21 and G30 and both are par the course for a glock - lower recoil, light weight, lots of ammo!
Good luck to you, but I'd vote for the glock!