Looking at pros and cons between the two. The 6721 has a HBAR and the 6920 is a M4. 6721 is 1 lb. or more heavier. Any other reason why one is better than the other? Twist rate is different also. Any input?

$30.00 price difference

If it's an issue, the 6721 doesn't have front spring takedown push-pins; they're screw type, though this can be modified. I've always preferred the spring receivers because of the easier tear-down feature. Neither type effects the weapon's performance.

The 1:9 is compatible with a wider range of ammunition than the 1:7, which was made to improve accuracy with heavier rds (62 gr and over). I heard it was specifically made to handle the heavy tracer rounds used my the military. That said, the 1:9 is fine with heavy loads, and more flexible overall.

There are varying opinions on the virtues of heavy vs. standard bbls. Personally, I'd go for a fluted heavy bbl. over a regular heavy. But, if you're LEO, then modifying yours is definitely not the thing to do, for the somewhat obvious legal reasons involved. Word is that the M4 bbl. came about for one (or both) of 2 reasons:

1) Increased front-end weight for stability during full-auto firing.
2) Increased rigidity to prevent bending when troops used them for prying (The M203 cut seems to diminish this effect somewhat).

I'd personally go with the 6721, mainly for the heavy bbl. and the ammo flexibility afforded by its 1:9 twist. Either is more than adequate for the job, though.

My $.02