They are two of the best 45's on the market today. You've got a winner either way but there are differences. The Sig P-220 is stone cold reliable, lightweight, durable (durability is overrated for normal civilian use,ie I don't throw my pistols in snowbanks or run over them with my pickup), combat accurate, expensive and reeks of QUALITY. The Glock 21 is generally stone cold reliable (it wasn't for me), lightweight, very durable if you throw your pistols in the snow or run over them with your caddie or immerse them in salt water for sixty hours, combat accurate, relatively affordable, not made for "Mexican carry" (which I do alot), with a round in the chamber it is a holstered gun, period. Capacity is a non-issue in a 45 cal pistol beyond 6 or 7 rounds. Two fine pistols but two different pistols. Just my thoughts, J. Parker