On the outside the M&P15T and the AR-556 are similar rifles - flat top upper, 10" Troy quad rail & Troy flip-up BUIS, 16" barrel, M4 collapsible stock. Underneath they're quite different primarily because of their gas operating systems.
The S&W utilizes the standard, battle proven direct gas impingement system whereas the Ruger utilizes a proprietary 2-stage gas piston system. While retrofitted gas piston systems can offer some benefits in the AR platform there are still quirks to be worked out (some research online is recommended before choosing a piston operated AR). One of these, bolt carrier tilt, has become a concern for AR-556 users. It's believed that stacked tolerances may be the reason a percentage of the Rugers exhibit carrier tilt and unusual receiver and buffer tube wear as a result. One more factor worth noting - gas piston systems for the AR are relatively new and there's no one standard design. If a part in a piston system fails then replacement parts from that piston system manufacturer will be required to repair the failure (some are young coompanies with uncertain futures). In contrast replacement parts for a standard direct impingement gun are available everywhere and are standardized.
The M&P15T's barrel is free floating potentially increasing accuracy. The quad-rail forend is also free-floating and is tightly clamped to the barrel nut. The Ruger's quad-rail forend is affixed to both the upper receiver and the adjustable gas block via steel roll pins. The downside to this design is that direct contact with the gas block can cause the aluminum rail to become hot to the touch after emptying a few magazines.
I'm glad to see Ruger making AR's. I'm just not very confident in the product in its current form. I expect they'll work out the isues with time and experience.
That's my $0.02