I have tried both, and over time the 1911 grows on me. The G21(SF) doesn't. Both are fine pieces and will probably serve you well, but if you are like me, over time you will bond with the 1911, but the "new" will quickly wear off the 21.
1911:
Pros - Slimmer (*much* easier to carry, in spite of the extra weight). Better trigger. Handles a wider variety of ammunition (mine will feed empty brass, which the g21 will not). Fun to practice with.
Cons - Some require a break-in period or some fiddling to achieve reliability. Once you get there, though, a good 1911 should be as reliable as any glock under all conditions, i.e. 1,000 rounds or more between malfunctions.
G21:
Pros - Cheaper, sometimes by half. Usually works 100% out of the box.
Cons - Very fat. Almost impossible to conceal. *Much* pickier about ammo (e.g. no lead bullets). Not much fun to practice with. Also does not impress your friends.
Capacity isn't really an issue. With modern, 8-round mags, a 1911 holds 9 rounds total, compared with the G21's 14. If you can't get the job done with 9 rounds of .45acp, you've got bigger problems than you can address with a sidearm.
If I were unarmed and needed a new gun right now for a life-and-death situation, I'd probably grab a G21. If I were leisurely buying one to last me a lifetime, I'd spend the extra money and go with a good 1911, take the time to break it in, and practice with it.