The variable chamber, belt type baler will start making a bale from the center, depending on the pressure setting the density of the bale will be consistent as the bale forms and grows, they work best on dry crops like hay and straw, longer the stem the better the bale generally. They also require less horsepower, I've run an NH644 (4x5) with 60 pto hp. A fixed chamber, roller type baler (soft center) allows the crop to rumble around until it fills the chamber then density increases as the bale forms in tighter layers towards the outside. These balers work well in wet crops like oatlage, haylage and cornstalks. To run a 4x4 baler and be able to make bales that have adequate density you will need at least a 90hp tractor, the more the better. They also handle short stemmed crops better than the belt balers. I make mostly dry hay, straw and stalk bales so I run an NH 7060, 4x5 hard center baler.