To put it bluntly, Glocks are loved by rookie cops, SIGs are used by the Secret Service, tactical teams, etc. They are different levels of weapon.
Now everything I just said assumes that we are leaving the trigger design out of the equation. But of course to most it matters, a lot. The Glock is a simplified trigger for those who lack training in DA/SA or SAO and need "a trigger for dummies" (no offense intended - but most rookie cops aren't gun guys - and this is mostly a cop gun). Many like myself who are long time hangunners comfortable with any and all triggers, don't care for the stupid little dangler on the Glock trigger, and I find it pinches my finger a bit. I have no use for it and find it obnoxious. I also have NO problem pulling out my P220 DA/SA and centering a head shot at 50 feet with both the first DA and the following SA shot. It's just a matter of training and practice. But for many shooters, especially women and younger guys, or men with less extensive shooting backgrounds, having a consistent pull is an aid to accuracy, and I understand this. So first you have to be OK with the trigger design of whichever gun you choose.
All being equal though, I will say this. The Glock is better if your gun might be run over by a tank or left in salt water for a month, it's tougher. But for normal users the SIG is more of an accurate, precision weapon. Everything that makes the Glock reliable (and it is very reliable), comes from the lockup patents they copied from SIG Sauer when the patents ran out, so they work great.
Don't let anyone give the BS line about weight savings either. For instance, my folded slide P220 weighs LESS unloaded than a Glock 21, and quite a bit less loaded. Other models vary up or down a bit, but in general when comparing loaded guns (we do keep them loaded right?), the Glock is HEAVIER, not lighter than a SIG in most cases, and in any case weight is a silly issue since both are easy to carry.
My own experience:
Old school SIGs are rock solid, totally reliable, and sometimes display accuracy that is "rifle-like". My old West German P220 in 38 super could print groups around 1" at 25 yards and could outshoot any other pistol of any price point, and did so on a regular basis, heel magazine release and all.
My glocks have worked well, except for the model 20 I owned. After less than 1000 rounds the plastic began to peel off of the steel part of the magazines and the gun started jamming a great deal. It was sold. 10mm literally tore the thing apart in less than a year. SIG and HK don't make guns in this caliber, and are probably wise not to, but at any rate, there is my one Glock horror story.
For most new shooters, the Glock will probably work better for you right off the bat to be honest though, due to trigger pull consistency. However, there is nothing wrong with developing some skills and learning to shoot a traditional handgun properly.