OK... Here is my own summary from while back:
Square bales are more expensive than round bales.
Square bales: easy to move, deliver, and stock up (and feed). Round bales are HEAVY (plus you can't keep them in small sheds like I keep my small square bales).
Square bales - almost no waste. Round bales - LOTS of waste (they spread it, pee on it, etc. etc.). At least in my experience.
Round bales: you want to give botulism shot to the horses.
So the bottom line was for just 2 horses I decided to go with square bales...