Unlike others I have spoken with, I don't have a strong opinion, as I'm not 100% confident that I am doing the right thing.
I have all of my bees in double brood boxes. I do this because of swarm prevention - I feel that if the girls have plenty of room for the queen to lay, they will be slower to swarm. I don't have much time to spend with the bees, so if I can't get to them for a few weeks in Spring/Summer, I like to think that I have those extra few days leeway before they decide to swarm.
Others I have spoken with go with one brood box, particularly the commercial or semi commercial guys. They obviously save on costs and time by only having to go through one box, not two. Not to mention the ease of transporting hives. People with more time (and knowledge) don't need that leeway as they will be on top of the timing of swarms.
From my point of view, because I don't move the bees, what's the down side of double brood boxes? For me, it's just the need for an extra box per hive, which is no big deal at all.