fernset - speaking as a reasonably exerienced AR guy (there are Guru's on this site that kick my but up and around the corner, so please correct me if I am wrong)
The M&P is a good quality rifle, it uses direct impingement which means that the gasses from firing go through a tube and push DIRECTLY on the bolt to unlock it and cycle the next round. This leads to a fair amount of carbon getting blown onto the working parts of the gun. This is how AR's were designed and have been operated for about 40 years. S&W's, while built as quality fire arms, do not meet "mil spec" or "military specifications" on what materials are used and how some of the parts are put together. This is the case with probably more than 90% of the civilian AR's out there however, and like I said, they are still a good quality fire arm and I would not expect them to ever give you any real problems.
The Sig is a "piston operated" weapon, instead of the gasses from firing pushing directly on the bolt, they push on a piston, which in turn pushes a rod against the bolt - cycling the weapon. You drastically reduce the amount (but not eliminate) of carbon and other by products that accumulate on the working parts of the gun. The other advantage that some (including the Sig) have is the ability to change the settings on the gas "regulator" so that you can allow more or less gas to push on the piston based on how efficiently the weapon is functioning. AS I UNDERSTAND IT the Sig is made of "Mil Spec" parts, and Sig does have the reputation though its SIG556 ( a similar platform designed by Sig) as building some of the highest quality piston driven 5.56 caliber assault rifles in the world.
I and also (rightfully so or not) a little skeptical of the "frankenstein" AR's out there where people put together many different brands of parts. I prefer all the operating parts to have the same level of quality control (for better or worse) and have a gaurantee for the rifle as a package.
Most of this is my opinion, and as such I am sure many will disagree.
However YOUR rifle comes down to YOUR preferances, some of which I may agree with, others, not so much.
In my line of work I will be planning on putting 100k rounds through this baby over its life time, and many of those rounds will not be in the best of climates. I am looking for a battle rifle, and until some one shows me something better.... well I think this is the one for me!