Will you use open sights, red dot, EOtech, scope? If you will use open sights get a 6520, no worries about the carry handle going loose. If a sight aid is what you want then get a flat top such as the 6920, optic it, and put on a good set of back up Iron Sights.
I'd stick with 5.56, you ALREADY have a pistol caliber weapon so go for the much more powerful rifle round.
The 6520 is a light little kickass AR. Very handy.