1.5" and you can't change your mind on a handguard that requires a new nut...Or, you can take a Dremel and chisel to remove and then reweld a new FH - Bottom line it's a personal preference, and it will become a HEATED one quick in this forum. There are true believers for both sides.
You lose 1.5" on the barrel (roughly). If you want a physics problem done to estimate how it is actually in weight with momentum versus "feel", there is probably someone that can muster up the equation. But I will say that unless you are talking tactical advantage - It's negligible unless you have muscle issues. The difference between a 26", 28" and, 30" shotgun barrels with skeet shooting makes quicker swinging guns with each step going shorter. However, there is a point that the speed in barrel movement becomes an issue with overswing and reflexes of the shooter versus a positive, smooth swing. I'll be honest, I NEVER noticed the difference in weight unless they had skeet tubes that ran the entire length of the barrel. Trap shooters like the longer guns - 30" and 32". This makes a more stable gun for shooting downrange, and they don't swing as much for shots - Longer sight plain. If you are going to use the gun for hunting (5 round clip for most states), general range, and home defense - 16" is a great compromise for length, stability, and keeping the gun modular which is the primary reason for this weapon.
I understand the tactical advantage of getting one out of a HUMVEE. I can't talk logically to a police car. However, when will the average person EVER do this??? As far as room clearing - #1 you need to be trained to some degree. 1.5" will not make much difference either way. If it was the difference between a 16" and 10.5" barrel, then there is a difference. You can get used to whatever is issued to you, so you can get used to whatever you build. Also, if you are using it for home defense, your adrenalin will overpower any issues with the 10 oz of weight or so.
If the length is such an issue, then one should fill out the forms and pay $200 to get the SBR stamp, put on the 10.5" or 11.5", and be done with it.