G'day Tecolote, and et al.,
When I was shopping for a 45 acp both the G-21 and the P-220 were strong considerations for me. I must admit that I was slightly bias as I owned glocks in other calibers. I ended up getting the Glock as it met my needs better for fun shooting and carry during the outdoor treks about AZ.
I've found both to shoot well. Accuracy was roughly the same at 25 yds with the various ammo types that I've tried.
Where I found the discriminating factors were the following:
Standard Capacity of 13 + 1 goes to the Glock.
Size and weight, again went to the Glock in my case for ease of carry, even with a full load.
Trigger, this one is won by the SIG. The smooth trigger for the traditional DA/SA manipulation was impressive. A true joy to operate. The Glock trigger is just functional. (note: installing a Glock # 3.5 connector provides a lighter smoother trigger with less stacking at the end, nowhere near the smoothness of the SIG)
Appearance, this goes to the SIG for aesthetics as it does have a classic look. The Glock just looks like a Glock. I have had several friends who own SIGs complain about rust and corrosion. My Glocks have been out in the snow/rain with maintenance applied several days latter with no detrimental affects.
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. This was rather significant to me as I tend to launch quite a few rounds down range on a monthly basis. Many of them are reload which can leave lots of residue.
I was fortunate to have access to armorer guides for both pistols. The Glock was just so easy to deal with compared to all the additional springs and pins of the SIG.
Please don't misunderstand, both pistols are fine examples of engineering and manufacturing. Both of them represent weapons that are completely suitable right out of the box for both casual and serious duty. In my selection criteria the Glock was my clear choice.
I've found that SIGs tend to instill an emotional disposition in their owners. Glocks are perceved as just functional tools. Best of luck in your decision and purchase! Both are good choices if you can afford both. Cheers!