Sig does have a polymer-framed pistol. Polymer is tuough but take a knife to a polymer frame and then to a steel or aluminum frame and tell me which is tougher, there's always trade-offs. Most of the Sig slides are now milled from stainless steel. The older carbon-steel slides have a weak bluing that does come off easier but won't rust unless you neglect it.
The stainless slides have a new finish that is much better. In addition, Sig parts are drop in and several of the parts are interchangeable between models. Plus, it still functions if there's a now non-existent frame crack problem (a "problem that's been corrected" or a broken or missing trigger bar spring (a "problem" that has also been corrected").
In regards to why there are more Glocks used by LE, there are many factors. Price being a huge concern. Plus, it's not like LE get to choose what they use, someone else makes that decision. Moreover, don't forget that many LE aren't "gun" people and the only gun they ever have/had is the one they are issued.
An LE friend of mine attended several training courses held by the FBI in the late 80's and early 90's. Back then, the FBI told everyone who attended that the Glock just wasn't up to par and was basically junk and the basic guns to go with were Beretta, Sig and HK.
Now, they issue the Glock as do other agencies. Obviously, Glock made some improvements (2nd, 3rd generation, etc) and was able to offer a price that was attractive. Plus, with it's simplicity, it was probably easier to teach new recruits that have a hard time understanding how to use a decocking lever.
Don't get me wrong, I think it's a great gun with a good trigger system/concept, and few parts that one can easily install himself/herself. I just think that their ergonomics aren't too hot. One day, I'd consider getting one Glock, the 20 just so I could have a 10mm.
In the long run, I think that all the well-made guns from the big companies like Beretta, Sig, Glock, HK, etc will stand up to abuse. As to who's the best, the nod probably is going to go to Glock but then again, who's done a real scientific study of it and then again, is it that important for weekend warriors and hobby shooters who will probably never wear the things out.