I voted red dot. As a life long Scope guy, this is quite a departure for me, but recent results has change my opinion. I have owned a AR-15 since 1968, but have not been a fan of the 223 because of the weight and hunting ability of the light weight bullets. But you can't argue with the lower cost of shooting 223/5.56 than any other centerfire cartrage. And I do shoot this round more than any others.
While you can shoot it longer, I feel it is an effective 25 to 150 yard round, maybe a max of 300 yards for takedown power. For longer range the 243, 308, 270 and 30-06 with heavier bullets, I personally feel would be more effective.
So with a 25 to 150 yard range in mind, the CQB effectivenes of the 223 would lend it's self to use of a quick sighting red dot for easy of use and fast on target results. And in this respect, the red dot has proved quite effective in it's results.
Each person will have their own personal comfort level with what they use, for short ranges I feel the red dot will be the best choice, for longer range shooting, I would use one of my bolt action rifles with a large scope on it. Just depends on the type of shooting you will be doing.
Jim