Oh the little devil on my shoulder is screaming at me. I'm a Sig fanboy and I hate Glocks. I figure full disclosure is only fair.
Now first we need to know what is it's intended use?
In general, I'd lay it out like this.
Both Sig and Glock have a huge following. Glocks are cheaper and Sigs are more expensive. Both enjoy a good reputation, but I think Sig edges out Glock there. I've owned just about every Sig model out there and love them all.
I'd easily say buy the P220, but in reality you need to rent/borrow each one and shoot them. How do they feel in your hand? How well do you shoot them? Which one do YOU like best as you handle them.
The P220 is considered the Cadillac of .45's in some circles. The Glock IMHO is just a tool.
I have however seen a guy shoot a great series of stages in IPSC competition with a Glock.
Would you buy a car without a test drive?