You're comparing a sight to something that goes on the barrel?
Well, in nearly every game I pick a suppressor so I'm not seen on the enemy map. I never pick the acog scope because I'm always running around - it's useless when running around. I usually leave the iron sights on if they aren't too ugly or I'll slap a red dot sight on.
I would say suppressor. It keeps me off of the map, decreases range a bit, which because I'm usually running around, doesn't matter to me. And I like the iron sights.