I have a couple of SW MP 15's .
- MP15T (tactical) full quad rail
Model M&P15T - 5.56 mm, Tactical -a standard AR , SW's version of an M4 ..
Model M&P15 Rifle .
I also have a Colt SP 6940 M4 .
I'm a fan of the AR platform , enjoy shooting them but am no expert .
That said , I've put probably a couple thousand rounds thru each of the SW's and only a couple hundred thru my new Colt , non have failed in any way . I only use brass cases , havent and dont care to try steel cased ammo , even though many have no problems with steel cases ...I dont care for them.
The only difference between the ColtM4 and SW is the mag well. The SW appears to be beveled to some small degree, mags load in easier, other than that its all mil-spec . I believe SW is a mix of Stag parts , not sure which components but regardless the quality is there.
Do some more research on here and some of the AR sites , but in my opinion you wont go wrong with a SW . Also buy now , that prices are back down from orbit before the next election madness kicks in and they go up again. I bought just prior to the last presidential election and easily paid a couple hundred bucks over the current retail price.
Good Luck and Merry Christmas .