6920 is the M4 type. Has the M4 barrel at a legal 16". That barrel has the grenade launcher notch and is thin under the handguards. It also has the double-heatshield oval shapped (and thicker) handgaurds. The twist is 1/7. Also has an F-height front sight.
The 6721 is pretty much the same thing except it has an HBAR type barrel which is heavier under the handguards, and it has the single heatshield CAR handguards. The twist is 1/9. Not sure about the sight, but since it is an M4 flattop upper, they probably have an F-height on it.
Being an owner of a 6520, I like as light as possible. So the 6920 would be a better choice for me. Also, it has the nicer handgards which will keep the hands more comfortable during rapid fire. The 1/7 twist guarantees you the ability to shoot everything up to 77gr (basically all factory magazine length ammo made). The 1/9 of the 6721 will limit you to 69gr at the most for guaranteed performance. If you get lucky, the 75's might work, but they might not.
If you don't rapid fire or are planning on changing out the handgards AND you're just looking at shooting 55gr and 62gr ball and possibly 68 or 69 for "business" ...the advantage of the 6920 quickly goes away with the exception of being more handy (lighter).
Me personally, I'd go 6920. But no matter what, these LE Colt rifles are the absolute top-notch AR's you can buy. You can't go wrong.
