I've had a 9mm P226R since 2008. It had a couple stove pipe jams in the first hundred rounds. I used it in a defensive pistol course a while back. It was a rainy day and plenty of mud and dirt got into the gun. While it had a couple hang ups under those conditions so did everyone else's gun that day. This specifically includes the instructor's Glock 19. I suspect that reliability under bad conditions is about equal between the two. SIGs do like to run wet - I think this is more of a preventive maintenance concern than a reliability issue. Some SIG owners have experienced issues with the aluminum frame rails wearing out over time - proper lubrication (grease is advised instead of oil) should limit this. I have not had this issue with mine, however.
The SIG finish is probably one of its weaker points - it just isn't as durable as the tennifer finish on a Glock. I think it gives them character. Finally, although my SIG has held up well, I doubt that any gun is as durable as a Glock.
It may or may not be worth the extra money - I paid $750 for mine. Today they may be priced higher. I think it's a better gun than the Glock 17. However I'm not sure it's $250 better and for me it sure isn't $350 or more better. It's a good gun, but not a $1,000 gun if you ask me.
Magazines are available at larger retailers (I've seen them at Cabela's and Sportsman's Warehouse, anyway). Parts and holsters you'll likely need to order online. However, these haven't been difficult to find there.
I'm sure you'd really like a P226, but definitely get your hands on one before you buy. The reach to the trigger and decocker may put you off.
Just so you know, I'm strongly considering a Glock 19 for my next purchase. As great as the SIG is, the high cost of the gun and magazines along with the weight remain problematic for me. I'm also looking for something that is simpler and as indestructible as can be engineered today in a handgun. I'm willing to give up a little performance and style to get it, though.