I have heard that slant is a better position for the horse. Probably based on the same study mentioned already.
My personal reason for getting a 3H slant is that Slant is the only 3H trailer I could find that was bumper pull. If I was looking for a 2H, I probably would have bought a straight load with dressing room. For 3H, slant was the only viable option.
I have a 16hh _very_ beefy QH (looks like a draft mix) that weighs in at >1300#. My other horse is 16.1hh, >1200# and very "long" necked. Both fit easily in the trailer. In fact, the slant gives them more "front to back" room than most straight loads I have seen.
Yes, the third compartment seems tight, especially with the fold out tack area. When I need to put a larger horse back there, I just take out the tack divider and the area is bigger than the other two.