Wow, great minds all think alike :)
I was also deciding between building, S&W M&P Sport 2, and AR556. After thinking it over, decided that a smart thing for a total AR noob to do would be to completely eliminate the issues of my own making by going with a pre-build rifle.
At first, wanted to get the Ruger, primarily because of all the hype about the twist rate. But have seen a bit too many negative reviews with regard to quality issues affecting accuracy. While I am 99% sure this was blown way out of proportion, I didn't want to take my chances.
Finally found an S&W for $499 + free shipping - couldn't resist that. Bought it last Monday, received on Thursday, took to the range on Saturday. 100 rounds of Federal FMJ later, I am a very happy customer. Not a single hick up, a pleasure to shoot, accuracy far exceeding my own. It does have a cosmetic blemish on the upper receiver, starting above the trigger catch and towards the barrel the finish looks a bit rough, so I may yet see how good the S&W customer service is. But overall, I feel this was a great choice, especially considering the price.