I like the idea of the aimpoint+3x mag because I can flip the mag to the side and use the rear sight.

I like the acog better because it's cheaper than the aimpoint+mag combo, but I don't like that it blocks the rear flip sight.  

Thoughts?

One's gotta flip up, the other needs proper eye relief.

You're gonna get one of the two to work at any given time... not both.

could you place the rear sight in front of the acog?  or would that be silly?

i have seen people do it. looks a little different. if i understand what your asking though, you wont be able to co-witness your BUIS and ACOG, no matter where you place the BUIS.

Magnified optics don't co-witness.  

You will have to remove the optic to shoot irons.

Like the others have said, you cannot co-witness with a magnified optic.  You will need to remove it.  Don't worry, there is VERY little chance that the ACOG will fail and you will need the BUIS.

One lifts off and the other flips to the side... what is the difference?

That's exactly why I decided to remove my BUIS to get proper eye relief on my ACOG. My AR is primarily a range gun, so iron sights are simply not required.

Marines in Iraq who are issued an ACOG remove the carry handle and stow it in their pack and use the ACOG in combat without a buis. I've learned to "let go" of my fear and run without one as well. I use the LaRue RCO ACOG mount and have mine set pretty far back because I'm not comfirtable with the tip of my nose on the charging handle. I like my nose about 2 finger widths from the charging handle.



With a red dot though, you are introducing a battery into the equation, even if you have a 50,000 hour lifespan with an aimpoint, I still insist on a functioning, dialed-in set of back up iron sights with aimpoints and eotechs.

This.  I don't understand the debate... no matter what the ACOG has to come off to employ the BUIS.