I have the M&P 15T and am very satisfied. I wanted an Optics ready, flip-up front and rear battle sights, and a free-floating rail. The 15T has all that.
I added an EoTech sight, a Hogue grip, a one-piece sling and it all functions perfect. I have no problem hitting the 500 yard target with this set-up. Have put around 2000 rounds thru it and it functions great. I did have a couple jams in the beginning, but I switched lubes from CLP to Slip 2000 E.W.L (Extreme Weapons Lubrication) and it has been the best lube I have used. It stays "wet" without the lube drying up and have had zero problems. I have also used different magazines and really like the Magpul P-Mags($14), CMMG Mag with stainless follower with braided spring($18), and the Tapko($14) mags have all function flawlessly. They are very reasonably priced as well.
Whatever configuration of the M&P 15 you decide should be based on what your needs are. These rifles are quality and are outstanding shooters.