The S&W M&P is a nice rifle. BUT, I would go with the SIG 522, if you can find one. The mags used in the S&W are proprietary. The SIG uses the more common Ceiner type and the upper is aluminum vs the plastic. But as you said, you want an AR type and the S&W is much nicer than the converted airsoft guns like Umarex and ATI. The S&W can use regular AR trigger groups so you can put match triggers in them if you wanted. Haven't heard much about the Mossberg offerings, they look kind of odd.
Not sure what your budget is but if you already have an AR, I would go with a dedicated upper and you can use your lower that you already have. If you just want to plink and accuracy is not super critical you can just buy a conversion kit that replaces your bolt in your AR. CMMG makes a great one, they also make dedicated uppers and complete guns. There are other manufacturers as well.