Hey RC601962, I'm only going to bring up the points that I considered when I was in your situation. For your needs, I feel that the RDS with a magnifier on a flip mount would be suffice. Without the magnifier, using both eyes open will be no problem. The only time you won't be able to use both eyes is when you use the magnifier. The same can be said for the ACOG's because the lowest magnification they have is 1.5x on some of their models. Some may be able to use both eyes open at 1.5x and some may not. The only way to find out is to try for yourself. I know with my Aimpoint 3x, I can't use it with both eyes open. About magnification, with the ACOGs, you're only going to have 1 option. With the RDS and magnifier combo, you'll have 2 options...
With regards to weight, I think the ACOG will weigh significantly less than an RDS and magnifier combo. For the M4s, alone, at 9.3oz, it already weighs a little more than twice the amount of the ACOG TA26S-10 (4.5oz). And with the factory mount, the M4s is already at 11.8oz. So, this is something you'll have to decide on your own.
Lastly, with the price, I think when you get a good quality RDS (Eotech or Aimpoint), magnifier and mounts, the price will practically be the same as that of an ACOG and mount depending on the model (cheapest ACOG model is $1010.00), or even more.
So with the points I have mentioned above, whichever route you choose, you'll have to weigh your options carefully. It's a big investment, so think it through and do your research. Either way, I think you'll be happy with your decision. Good luck!
-BigWil