Another price factor is: polymer or aluminum lower receiver. Unless heavily used in battlefield and you are worried about a tank driving over it. Polymer is fine. However I usually get aluminum. Since it's not much different in price. 7075-T6 is mil spec. 6061 is less sturdy. But again. They're all fine.
mil spec barrel twist is 1x7. Twist rate for 1x7 means that for every 7", the bullet twists once. Making it more accurate than a 1x9. This would be more of an issue for long range shooting and not most close range. (If you go with a less popular caliber like 300 AAC, most of those barrels available that I know of are stainless steel with 1x8).
What kind of sights are you using. For home defense- most likely you're going to use a red dot scope- or - use iron sights. If you want to save some cash- you can get one with an a2 front sight. This means you will only need to buy a rear sight. If you go with red dot- you don't need to go all out with an eo tech- sight mark is a good brand IMO.
For future customization- if applicable- should you decide to want a free float hand guard- an a2 sight is a pain in the butt to take off- (necessary to install free float)- due to the tapered pins.
Also- the "traditional" system is gas impingement. There's another system that's been out for a little bit now- that's piston driven (same system used by the ak)- you'll notice it looks slightly different. Basically- piston is cleaner- if u ran a rag thru the two different ARs that have just been shot the same number of times. The rag for piston would be cleaner. It also runs cooler. Piston operated are generally more expensive and from what I've read- relatively less accurate. Should a suppressor be something you want in the future- piston is less compatible.
There's so much info regarding an ar. I like the colt LE 6920 (mil spec). And bushmaster Orc (1x9, but has 7075 lower).