I have both the 6920 and a 556 pistol. Both are well made firearms and have been extremely reliable.
The 556 tends to run cleaner due to the piston design, but is a little more complicated to field strip and requires two screwdrivers to do so. The 556 pistol and 6920 weight about the same, the 556 rifle is 1.5-2# heavier, although if you put a rail on the 6920 you will eat up some of the difference.
The 556 pistol is very accurate despite the short barrel, so I'd imagine the 556 rifle would shoot very well.
I don't think you could go wrong with either.