Glock 19 Advantages:
4.75" height (bottom of magazine to top of slide)
Polymer frame to reduce weight
Shorter barrel, but long enough for good sight radius
15+1 round capacity
Grip angle improves concealability
Easy to control under recoil
5.5# pull from rest
Very short reset for follow-up shots
Tennifer finish is exceptionally tough
Genius of design is in maintenance. Anyone can learn how to take the gun completely apart to replace parts
Trigger job costs $25 for Ptooma Productions manual + a bit of polish and 15 minutes
Tons of accessories and gear
Spare parts are available off the 'net
SIG P226 Advantages
Aluminum frame and high quality
Double action trigger with single action second shot (run it like a Glock off the reset)
Very accurate
Longer barrel for increased performance and longer sight radius
15+1 capacity in 9mm
Grips may be changed
The Glock 19 is easier to carry. The overall height is less than the P226. The grip is the important part; this is the part that prints most frequently. While width does matter, the two guns are very similar such that this doesn't matter. Grip material does matter since some materials will "grab" at your shirt, changing how the clothing moves. Get thin grips for the SIG P226 (Hogue aluminum grips come to mind).
However, the SIG is a REALLY nice gun. It feels better. You will probably notice that your groups are tighter. However, for combat shooting, the difference doesn't matter. Also note that there is a higher price attached to the P226. You'll probably want night sights, thin grips, short trigger, short reset package, and a trigger job (Gray Guns is the way to go, but the work will be pricey. SIG does a decent job). These changes could be quite expensive.
Both guns are very reliable. I'm not sure one is better than the other. I believe the choice comes down to cost, fit and the level of concealability that you need. The Glock 19 is easier to hide than the SIG. Proper holster choice is essential for both guns.
Rent both guns at a range and put 200 rounds through each. At the end of that 400 rounds, you'll know which gun you prefer.