Pirate put it pretty well - I think I would rate the S&W at high/mid grade myself! lol. The higher end rifles, like Colt and Bravo Company are the ones that will be rockin and rollin 20 or 30 years from now with normal maintenance. The mid level rifles, bushmaster, DPMS etc will all be fine shooters and may last just as long, but when you start getting up into the high round counts parts will have to be replaced more frequently and you are more likely to start getting malf's due to screws coming loose and worn metal.
So in the short run they are pretty much the same - in the long run is where you are going to see the difference between high end AR's and Mid to low level AR's.