My dad bought our first hay cutting equip around 1975, so I have been baling hay literally all my life and most of that time we were also custom balers. We started out doing only square bales then move to doing both, but basically stopped the squares when labor become too hard to find. The size of our round baler has always basically depended on market demand. Back then (in our area), not everyone had a large enough tractor to handle the big bales or just didn't want to fool with them. I don't do that much custom work anymore unless you count helping out the neighbors.
Personally, I prefer the bigger bales as we can get across the field a little faster. Sometime over the winter we plan to trade up to a 5x6 baler since that's what our market is demanding now and that's fine with me. The plan is to bale 700+ 5x6 bales next year, but we all know what can happen to plan.