Guess I may as well chime in.... I have the Sport and love it. Great rifle for my needs. Put a Matech BUIS rear sight and mounted a red dot on it and that is it. Still pretty much stock. For $650 very hard to beat. Great value for the money.
However, as you climb the S&W ladder, in my opinion, you start to see that value erode some. You can still get good deals on the M&P 15 OR, and they are a good buy, especially when they are found in the $750 (or less) range.
Once you start getting into the higher priced S&W models and the $1000 and up range, there S&W might not represent as good as value to some. S&W tends to have more "upgrades", such as rails, Magpul furniture, etc... to their higher end rifles, but they don't follow the TDP. You will have to study where they deviate and decide if it is innovation on their part, or cost cutting. (Or maybe a combo of both, like melonite treatment of the barrel)
Buy what you can afford and what you like. If you must have a dust cover and forward assist, you shouldn't buy a Sport, because you won't be happy with it. If the pony rollmark makes you smile everytime you see it because it was your trusty friend "over there" then get one. If you want a midlength gas system, with Magpul furniture, and a 1:8 twist, melonite treated barrel made from 4150 CMV, with a couple of Magpul training DVDs thrown in, S&W has got you covered there....Many rifles out there, you have to decide what flavor you like and are willing to pay for.