On many ballistics charts a lot of loads only lose 100-150 fps going from 20 to 16 inches. Not a big deal when the bullet's still going 2700-3000 fps. I'd go with the carbine length. Now when you go under 14.5" the difference gets substantial.
Of course you can put a longer fixed stock on a carbine if you want. Or just extend the telestock.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek