I own both so I amy be able to comment.
All of my guns wear Aimpoint optics. I have a pair of BCM 14.5" ELW-f barrels with 13" KMR rails. the only difference between the two is one had an Aimpoint T-1 and the other an Aimpoint PRO.
My 6720 has an Aimpoint PRO.
When thinking of my reply to this, I will compare the Colt and BCM that both wear the same optic and light, so that the only difference is the actual gun itself.
I used Colt 6920’s for a long time as my “all around go-to” gun. even carried one at work as a Patrol Rifle. They are still a solid choice.
The 6720 brings everything to the table that the 6920 does, just with a gun that is more balanced and “lively feeling” due to the barrel. For the all around use being HD and farm usage, I think the 6720 is a solid choice. You are talking about something that will be carried a lot. When shots are fired, we’re not talking about multiple magazines worth, but a few at most. So the thin barrel heating up is a non-issue.
As much as I like the Colt 6920 and 6720, I now use the BCM guns I described as my all around go-to guns. This is due to my long arms and style of shooting that favors the 13" rail. I prefer this setup when deploying the gun in the urban environment that I work in. When shooting while moving and shooting multiple targets such as a shoot house type of training, I just perform better with this gun than I do with a carbine length gun with a fixed front sight.
The downside of the BCM uppers I am talking about is the cost. I have about as much money in the factory built BCM uppers as what I can buy a new 6720 for.
I own a 6720 that I have outfitted with MagPul’s new SL hand guards and stock. This keeps the weight of the gun down, and still permits me to mount a white light. Whenever I travel on vacation by car (I prefer not to fly if at all possible) this 6720 accompanies me. I call it my “vacation gun”. I do this because if something were to happen to the gun, I have less money invested in the gun. Yet it is still reliable enough that I would trust it, should the need arise.
IF the price of both the 6720 and a BCM gun with a ELW barrel and a KMR rail were the same, then I would say go for the BCM. BUT they are not the same.
So to the OP, it depends on wether the price difference is worth it to you, considering your needs.
The upside to this discussion is that given that you have limited your choices to these tow guns, you really can not go wrong. Either gun is an excellent choice for your intended purpose.