Re: ACOG or scope?
Id go variable power scope were I you and drop the acog on another AR, assuming you already have a nice precision job in the safe.
BUT it depends what you want to do. If you are trying to shoot groups at range, then I think you would be better served by the precision offered by the variable scope. Hunting or shooting small targets would also be easier and better suited to the variable glass.
If you are bangin steel, varmints, or something similar, I think you would be well served by the ACOG.
I shoot a 4x acog with the chevron off my ar15 rifle and it is great. I have not shot it past 500, but I dont see why I couldnt hit my ipsc steel beyond that. That said, it is really not my preference for group shooting etc. Otherwise, it has great eye relief, it is compact compared to a big variable, and the tritium/fibre optic lit reticle is great too.
For me a big decider would be how good of a deal you are getting on that ACOG or if I had another scope laying around. Soooo, both would work for ya IMHO.