If I were getting a carbine I'd go the 14.5" route.
For mid length go with the 16".
But honestly you should just pick which one you think looks better. I doubt you would be able to see a difference in performance.
And if you plan on ever changing your flash hider/compensator then definitely go with the 16"
As far as the gas block goes, if you can't find an upper that already has a low profile one, they only cost like $30 and they aren't too hard to swap out.
This post was edited on 3/17 at 4:41 pm