Re: Thinking about first AR...
Just about any quality upper with a good barrel will hold MOA with good ammo, if it's on an lower with a good trigger (at least in 223/556, some of the thumper AR chamberings open up a bit). Your Sig should be an MOA rifle if it's it got a good trigger. But as above, Sig is pretty proud of their stuff, as such, I think you can get more rifle for the same $$. Several makers guarantee MOA for that kind of $$. Even my old truck gun shoots MOA, and it's just an old RRA Coyote Carbine upper. Like any good rifle, you're mostly paying for a good "go" button and tube.
I like things that go "bang"... I love things that go "BOOM!"