I've owned two examples of each, shot thousands of rounds through them, carried them for years, and have seen dozens carried and used.
SIG - PROS: more accurate, better grip feel, better pointability and more natural grip angle, more easily concealed.
CONS: heavier felt recoil, 8-rd mags not reliable, not as durable. SIG P220 frames AND slides have self-destructed in as little as 2000 rounds, usually by 6,000 rounds - - catastrophic cracking. SIG beefed-up the frame and slide a few years ago, which only served to put off the cracking for an additional 2,000 rounds or so. Stamped steel parts such as decocking and slide stop levers bend and break. Trigger bar and decocking lever springs break. Roll pins that retain the breechblock in the stamped-steel slide crack frequently. SIG service guarrantees on these problems have varied over the years, from pretty good to awful.
Glock:
PRO: high mag capacity (pre-ban), reliability, decent combat accuracy.
CON: huge size for concealment, too large grip, poor pointing, company is known for weasily customer service at the retail level.
Triggers are a draw, as they are totally different, depends what you like. SIG is VERY heavy DA and a nice 3-4 lb SA. Glock is a decent one-pull SA-like 5-6 lbs.