I agree with what Fred said about LW vs. govt profile - the LW is much more maneuverable and you will notice a big difference because of balance more than the 6 oz. Even though the govt profile is more popular it's mainly because people don't compare the two. Take a look at pictures of the two profiles and you'll see that the govt is thinnest under the handguards and thick out past them. If you think about it, that makes no sense, except for the stories of GIs using their barrels as crowbars.
Sorry, the DD sales guy is wrong and many people in the know are demanding LW more and more every day (which is probably WHY the LWs are OOS). I suspect MJ will be chiming in soon as he has definite opinions on this too.
Don't get me wrong, buying the govt profile isn't wrong for everyone but try to get to a gun shop NOW and compare the two profiles.
As far as 14.5 vs. 16: there is a definite advantage to the shorter length IF you're 100% certain which muzzle device you want and will never change it. You do end up with a longer barrel with 16" + MD but the muzzle is threaded and you can make changes easily since the MD is not permanently pinned. I've gone with 16" on both my ARs but many go the other way.
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