The Carbine gas system is fine for 14.5″ barrels, but if you are considering a 16″ barrel, I highly recommend a mid-length gas system.
The mid-length is much better suited for a barrel of this length. Not only is it more inherently reliable (and closer to the original 20″ rifle length gas system that the AR15 was designed for originally), the mid-length system does less wear and tear on the rifle, has an increased sight radius, and produces slightly less recoil impulse than a carbine gas system.
Unfortunately, the mid-length is not as common and easy to get as a carbine length 16″. The bright side is, they are becoming more popular and common, and it just so happens that my favorite AR15 brand is producing them.
Bravo Company Mid-Length Uppers Not only are the BCM uppers well priced, they offer milspec quality such as HPT/MP Inspected proper MIL-B-11595E barrel steel chambered in 5.56, M4 feedramps, chrome lining, and a 1/7 twist.
In short, you get Colt quality for a lot less $$ and in a variety of different configurations.