For the cost, I'd keep the 6720 as a mule upper (training, other uses) and pick up an SBR upper (and run it as a dedicated suppressed unit maybe - at least set it up around being a suppressed SBR capable setup).
A 6933 upper with a Centurion C4 Cutout rail would be my SBR upper of choice if you want to go pure Colt. You then have a quality lower, and two uppers. One 11.5" you can use with or without a can, a a training/portable/high round count/lightweight 6720 upper to use in the interim, and continue using.
If you want outright best can for part time on part time off, Surefire without a doubt. For a mostly suppressed upper, the AAC is the best value hands down, though an 11.5" opens up a full range (to a Titanium dedicated thread on, to an old school OPS Inc reflex can) For the money, though I'm not an AAC fan, it's incredibly hard to beat a 6933 and AAC 91t M4-2000.

