With a good bullet that is designed to kill bad guys, you or them will never know the difference. The 21 is a full size pistol and will be heavier and harder to conceal. Pick the one that you can put hits on target.
Are you concerned about carrying concealed? I have a Glock 23 and it is very difficult to conceal during the warmer months. I ended up purchasing the newer Glock 42 (.380). Here it is compared to my 23.
Rumor has it Glock will be coming out with a new single stack 9mm at the next shot show.
Glock 21 is very difficult to conceal. I have one and love it, but seldom carry it due to the need to having to be "dressed correctly" to conceal it. I can carry my 23 will little problem. 1911 compact is even easier to carry.