I own a Ruger AR556 and have shot a number of Smith and Wesson M&P15 Sports or Sport II carbines. I would consider both to be very solid carbines that offer great value for the price. I also own two other more expensive ARs including a Colt LE 6920. The Colt does not shoot any better than the Ruger. I have seen both the M&P Sport II and the Ruger AR556 advertised for prices at and under $500 in recent years. I personally went with the Ruger because I preferred the 1:8 twist rate barrel on the AR556 to the 1:9 twist barrel of the Smith. But either would do fine if you plan to only shoot ammunition with projectiles of 70 grains or less.
It really depends on what you want to do with the rifle. If you are going to be target shooting primarily and just want to learn the platform, you do not need to shell out a grand for a BCM. I assume you don't anticipate spending days in the rain in the jungle, crawling through rice paddies, or engaging in sustained firefights in which hundreds of rounds are shot in a few minutes. If so, there may be no practical advantage to paying for a chrome-lined or melonited barrel.
I have not shot a Springfield Saint. On paper, it has some features that might warrant the additional cost over the Smith and the Ruger, namely better furniture (including and M-LOK compatible hand guard) and a mid-length, rather than the carbine-length gas system of the other two. But I would not lose much sleep over the gas system. Another consideration if you are willing to pay a bit more than the cost of the AR556 or M&P Sport II would be the Ruger AR556 MPR. This offers a free floated, M-LOK hand guard, an 18" nitrided, 1:8 twist barrel, rifle-length gas system, a much better trigger, and better furniture, but comes without any sights or optic so you will have to add one or the other, or both. It is a very solid value for around $600 and would probably be my number one pick in that price range.
Also in that same general price range would be an Aero Precision. Brownell's often has sales on bare Aero Precision rifles to which you add a butt stock, free-float hand guard and optics. I would also look at Palmetto State Armory. They offer a very wide variety of complete AR rifles or separate AR lowers and uppers offering a wide array of barrels and other options.
Of course, if you do want to shell out a grand or more, you can have your choice of some very nice rifles.
