while their is a "real" difference it is still very very little.. - unicast to 5 people or multicast to 5 people you are still sending the content down the line to 5 people.. sure you can strip it down and not have a much over head.. but the content is what is the large part and you are still transmitting it..
It's not overhead... If two neighbors are watching the same show and are both on U-verse, the content appears on the majority of the network once, not 5 times (only being duplicated when network topology requires it). I'd call that more than overhead. If you and your 4 neighbors both request hulu broadcast of the same show at the exact same time, it's 5 times the content size.