I would say that given that the breast is generally something you are born with and is genetically predetermined in size (unless you purchase the size/shape you desire) and the butt is something you have to work on to keep up it's shape, I would go with backside.
I prefer the backside because I've never seen breasts as something that you drool over. My opinion is that they're made to feed kids and are there just for that, not to be stared at. Boobs are beautiful, no lie, but I like healthily round backsides more.
I would say that given that the breast is generally something you are born with and is genetically predetermined in size (unless you purchase the size/shape you desire) and the butt is something you have to work on to keep up it's shape, I would go with backside.