[Flakbait]

Buy quality, buy it only once. Buy cheap...you might regret it.

If the money is tight, get a Stag, DPMS, Smith and Wesson, Bushmaster, etc and don't think twice. Few people make a bad AR these days due to intense competition.

If you can afford to pay a small $100-300 premium, look at Daniel Defense, LMT, BCM, Colt. They a higher level quality product which can withstand more abuse and keep on working.

There is probably not much difference to the average user but you can be assured of a quality product that a Tier 1 operator would not hesitate to take with him in harms way.

If you are like most people, you will eventually accessorize it and try different things so I would stick with the basic flattop design until you know exactly what you want. Handguards/Rails are cool but they add bulk and weight. I would try the basic configuration until you know exactly what you want. Magpul makes an inexpensive plastic handguard that allows you to attach some rail pieces for attaching accessories.

Given a choice since you are starting with a blank slate, I recommend the midlength gas design for 16 inch carbines over the carbine length gas tubes. There is not a big difference between the two designs, but the midlength design is a tad bit more reliable due to lower gas pressures and there are newer handguard designs and rails coming daily for the midlength gas tube AR.

The only advantage of the 20 inch AR is a higher muzzle velocity useful for long range shooting. The 5.56 really runs out of gas at 600 yards and you need a pretty fancy setup with free float rails, good riflescopes and good shooting ability (reading wind) to achieve meanful hits at that range. I think a 5.56 bullet traveling at 600 yards acts like a 22LR at point blank or something to that effect.

Since most ARs are rarely shot beyond 200 yards, the 20 inch barrel offers little for most folks.