I’ll echo some of what has been posted and add a few of my own thoughts on the matter.
When S&W released the M&P15 product line Stag was involved in production. There were a few wrinkles with these early rifles. Shortly thereafter S&W tooled up to manufacture the entire M&P15 line in-house (as posted earlier the barrels are Thompson - owned by S&W) save the after-market goodies from Magpul, Troy Industries, etc.
I don’t have any personal experience with Stag Arms AR’s so I cannot comment on their quality / reliability. I know someone who owns a Stag AR and he’s very happy with it.
I do own a S&W M&P15T and my personal experience is all positive - 2,000 rounds and a variety of .223 and 5.56 ammo so far without a single malfunction.
As Lethal Dose posted consider your intended use for this AR when making a purchase decision.
‘The chart’ - there’s useful information to be found on ‘the chart’ to be sure. You’ll get differing opinions as to how critical some of the listed specs are. A little reading with your intended use in mind and you should be able to decide how much of ‘the chart’ is important to your decision.
I’ll express my opinion one one particular specification (I’m not suggesting you adopt my perspective but rather encouraging you to make your own decision). I’m quite sure that I will never push an AR hard enough to have to consider the difference between a 4150CMV steel barrel and a 4140 steel barrel. It’s something I would consider in a full auto / select fire carbine (which I cannot own / discharge in my state :mad: ) assuming I had enough money to dump through the bore to cause it to heat to a point where the difference in steel would make a difference. I’ll never push a 4140 barrel hard enough for it to matter. Ever. With that said the mil-spec barrel steel is a ‘chart’ spec that I’m not concerned with personally. Read up on this subject and come to your own conclusions.
Rifling - everyone has their own preference where rifling is concerned. Personally I chose to go with a 1:9 for my first AR as 90% or more of the ammo that I send downrange is going to be 55gr .223 factory loads. I should say that I didn’t go looking for an AR with a 1:9 barrel. The AR that most closely matched what I wanted came with a 1:9. 1:7, 1:8, and 1:9 were all acceptable options for this AR. My next AR will have a 1:7 barrel as I’ll be using it for shooting 100-500 yards and I’ll want to be able to use heavier (longer) bullets. Again - don’t let folks tell you which is best for you. Read up on the subject and draw your own conclusions.
A mil-spec AR is a great choice. What’s most crucial, though, is having a reliable firearm that best suits your intended use.
The two rifles you have linked in the OP come equipped with a few after-market goodies and this brings the price up a bit. The Stag 2T has a Samson Star carbine length free floating rail ($270 street) and a A.R.M.S. 40L rear flip-up BUIS ($100). The S&W M&P15T comes with a Troy Industries 10" MRF FF rail system ($210) and front/rear Troy flip-up BUIS ($220), not including the low-profile gas block or the labor to install it if your not willing or able to do so yourself. The 5 Pmags are another $60 in value. If you’re looking for an AR with a ff quad rail and flip-up sights you’re going to be looking at roughly $1,200 or more. If you can get a M&P15T for $1,200 you’re getting a good deal (any quality AR with $400+ worth of extras for that price would be a good deal assuming you want those extras).
If you don’t care about the quad rail, the ff forend, the flip-up buis, etc. then the Spike’s Tactical LE carbine or mid-length are quality AR’s for $800 (spec’s appear to be nearly identical to a Colt 6920). The Colt 6920 will cost about $1,200. Whether or not the Colt is worth the additional $400 is a subjective matter (i.e. to some folks the Colt would be worth the asking price or more over, say, the Spike’s LE while to other’s the Colt might be $400 more than they need to spend).
Again, as others have posted, there are other options to consider in the $1,200-$1,300 range. Look at everything available within your budget, decide what is (and isn’t) important to you, and decide from there. If you’ve already done so and you’re down to the Stag 2T and the M&P15T I would suggest going with the M&P15T - you’re getting more for your money with this choice IMO.
That’s my $0.02 