I own a few ACOGs in different flavors, but when putting together defensive carbines I keep coming back to the tried-and-true Aimpoint Comp M2/LaRue LT-150 combo. For me it's about geography as much as anything else. Where I live the chances of me taking a shot in anger at a distance greater than 100m is pretty much zero. While I've gotten pretty fast with the ACOGs over the years I can still acquire targets faster with an Aimpoint
I may be in the minority, but I prefer the eye relief and speed of the 3.5x ACOG over the 4x, despite the added weight:
ETA: Here's a typical carbine patterned for defensive/training use:
Since the vast majority of my training and practice consists of 0-200 meter shooting my carbines are pretty low-tech compared to many. I don't freefloat them for example, because at those distances I don't need to. The only thing missing in the pic above is the sling.