Hi Miles2014. You can't go wrong with either one. I'd recommend watching Hickok45 on YouTube. He has videos on both.
I ended up getting the Ruger, but I think I likely would have been just as happy with the M&P.
I will, however, say that there were several factors for me that swayed me. The Ruger just seemed to offer more features for the price. Regardless of whether they are useful or not, I like having a forward assist and dust cover. I also like the beaver tail pistol grip, the milled front sight/gas block and the threaded delta ring.
Lastly, there currently is a bit of a difference in price. The Smith can often be found for about $50 cheaper than the Ruger.
Looked at both when I decided if I'm going to get a 'black rifle' I better get one while I'm able. Tried both at the range, loved the Ruger but the shop that I do business at made me an offer on the Sport that was several hundred dollars below the Ruger. Couldn't turn it down. Only around a thousand rounds through it but it chews up and spits out anything I feed it from the crummiest milsurp to high end Hornadys . Love it.
AR556 has a 1:8 twist M&P has a 1:9. If all you want to shoot is light 55gr or 62gr M855, then I'd go with the M&P. However, the AR556 will shoot 55gr okay but will do far better with heavier bullets. They are pretty much the same in quality, fit and finish. There are few features that set them apart. The main differences are the twist rate and Ruger has a much cooler logo.
I started doing my homework on M4 type AR-15s right after Christmas (2014) and found several companies offer them in pretty comparable price ranges. I found the difference between the Ruger and S&W was minimal but slightly favored the Ruger .... dust cover, and forward assist. I actually wanted a 1:9 twist rate ... which is optimum in a 16" barrel. In March, I went to a gun show and found a dealer had a "gun show special" .... a Ruger AR556 with 100 rounds of ammo and an extra 30 round Magpul magazine. I bought the Ruger and am very happy with it. I may have been just as happy with a S&W or another brand but ... I'm a Ruger fan plus it was available when I had money in my pocket.
I found the 1:8 twist rate was a bit different than I expected, in fact my Ruger AR556 is more accurate with 55gr bullets than 62gr bullets by just a tad. My intended purpose for this little carbine is a short range plinker .... 100 yards or less. Due to my poor vision, the open sights just don't get it so I mounted a 3x Nikon P-223 scope. My configuration is exactly what I wanted .... quick to get on target, accurate enough for it's intended purpose, yet didn't break the bank. I realize, not everyone shares my same priorities or choice of brand or configuration. That said, I would not hesitate recommending a Ruger AR556 to anyone .... as long as they don't expect match grade long range accuracy.