Well I'd say if it were plainly up to skill, the reg scope would prolly need more skill in order to be used, the M40a3 with Stopping power is as good as the barret, or the R700. But if you shoot a person with juggernaut, the required place for them to be killed if they have juggernaut is if you hit them in the neck or head. Since stopping power and juggernaut cancel each other out. The real power of the m40a3 comes in and so u have to hit the neck or higher. It also takes more magnification time with the regular scope. The M40A3 with ACOG is very unique and has a feature that is seen in no other sniper rifles. It increases it power from 70 to 75. So it is a one shot kill from anywhere if the enemy shot does not have juggernaut. Also if they have juggernaut it may be easier cuz the magnification is more shorter than the reg scope giving them less time and easier to shoot people with juggernaut which is with Acog the m40a3 also needs a shot from the neck or higher. Of course Acog also has its downfalls which are that it gets a lot of recoil and and more sway but for adept players, it doesn't really affect them.
So if You are an adept player and have more skill than others or if it's a gigantic map then i'd say Reg.
But if you are kinda a noob or is a adept player but want the game to be a lil bit easier, i'd pick Acog