As a civilian, I have no need to mount a M203 grenade launcher on my home defense weapon, so I prefer the lightweight Colt 6520/6720 carbines and I swap the M4 handguards for the smaller diameter XM177E2 handguards. The aforementioned Colt carbines have lightweight barrels, sometimes referred to as "pencil" barrels. Internally, they have the same barrel rifling twist rate, same barrel material, bolt carrier, bolt, and other functional components as the Colt 6920/6940 series carbines.
The Colt 6921 is the closest thing a civilian can purchase to a real M4 carbine, but it has a 14.5" barrel and requires a tax stamp. The Colt 6920 is identical in every other way except it has a 16" barrel and does not require a special tax stamp from BATFE. The only difference between a 6921 and a M4 is the 3 round burst or auto sear components, which you can't legally purchase if you do not own a registered machine gun.
If you ever think you'll need to mount a M203 grenade launcher, get a 6920 or 6940 series carbine. If not, the lightweight carbines have thinner, lighter barrels. If you need a M203 for home defense, I'd hate to live in your neighborhood.
Colt produces their carbines and rifles to military specification. They are not the only game in town, the producers of the best AR money can buy, or the only quality product available at a reasonable price.
I personally own Colt carbines because the product receives the due diligence in testing and quality of workmanship suitable for a carbine you are staking your life on at a reasonable price. It's hard for me to justify the price of a carbine that costs twice what my Colt cost when both are accurate, durable carbines that go bang when you pull the trigger.
That said, Colt, Bravo Company Manufacturing, Daniel Defense, and Wilson Combat all make superb carbines that are suitable for home defense.
If you are overly concerned with durability, I would consider purchasing an AK pattern carbine from a reputable manufacturer, like Arsenal.
For me, the Colt 6520/6720 carbines have the right blend of accuracy, ergonomics, balance, weight, durability, configurability, and availability of ammunition/magazines/accessories that I consider best for my uses. YMMV.
If you absolutely must have a piston AR like so many others who think they're going to stop a tyrannical government or the Chinese from invading with their favorite pop gun, Uncle Sam has already done the T&E for you. Your choices are the HK416 or the SCAR 16S. Neither of those two carbines is lightweight or cheap, but both have more testing and engineering put into them than do other "solutions" by dint of the amount of our tax dollars spent on them.
Whichever flavAR (yes, I'm adding important new words to the English language) you purchase, do yourself a favor and purchase some PMAG's from MagPul.