These are my thoughts on why I bought a 14.5" upper (CMMG 001):
1. If it's good enough for our boys to use in Afganistan, it's good enough for me, and the army qualifies on the M-4 up to 300m
2. It's lightweight, and I am recovering from a shoulder injury.
3. It's cool.
4. The Phantom suppressor is awesome! It really works!
The length from the front of the FSB to the rear of the crush washer on mine is 4-5/8".
You might also consider DPMS's Panther carbine (upper) RFA2-PCAR which has a 11.5" barrel with a 5.5" suppressor, if you can find it. The drawback to having one of these pinned uppers is that you are very, very limited in the hand guards you can get, since you can't remove the suppressor, so you can't remove the FSB. Also, getting the CMMG that I got may be near impossible to get, since according to the guy I ordered mine from, there are no more of these uppers at the distributors.
Also the short uppers are more costly then the 16". If I hadn't decided early on that I wanted a 14.5" upper, I would have gotten a Spike's, because I love their logo on the top rail and the bolt, and they have a good rep, they are reasonably priced, and they come with a buffer to match their gas system. Next choice would be a RRA mid length A4, because they are also inexpensive and have a good rep.
As for the advantages of the 14.5" upper they are more expensive, harder to find, limited in hand guard options, slightly less accurate (but who gets a carbine for long range accuracy?), and you have to defend your choice to anybody who asks. If you are unsure that you want the short barrel, I don't see any real reason to get it, but I got mine because I WANTED it.