Re: M4 or 20"
I have both carbine and rifle and prefer the balance of the rifle. If you're shooting iron sighted, and the A2 are very nice sights, the rifle will give you a better sight radius. I don't do much house clearing and the only time I thought someone was trying to open my door, the Sig P220 cleared corners pretty fast, I suspect faster than any carbine.
ETA, I may be weird, but I never did like the contour of the M4 barrel where the supposed grenade launcher attaches to, and the idea of using a carbine with something mounted onto the barrel to scare birds off of trees just sounds too redneck for even me. A regular (“skinny”) or heavy barrel contour may cost you less money (in addition to looking better).