I think a lot depends on how large your multibox teams are. Personally, I always prefer to take 4 into BGs instead of 5 (unless it's AV).

I'd say that you'd be better off splitting the 15 people you get into 3 teams of 4-boxers and 1 tri-box team. The tri-boxer can be a roamer on maps like AB to effectively put 7 characters at a node, for example. I think EOTS will be a great rated BG as a boxer, simply because it's always been a nice map for a boxing group. I think it's easier to bottle up the enemy and 4-cap EOTS than it is to get a 5-cap AB victory (against premades).

Taking 3 5-box teams into any of the 15 person BGs will probably be much harder than four slightly smaller teams, in my opinion!