I believe the reason the 60" has the self leveling feature is due to it's larger size and weight over the 54". Conversely I don't believe the 54" requires the self balancing feature because it's size and weight don't require it.
Simply put, the self balancer on the 60" consists of a heavy tractor to MMM bracket that runs the length of the 60" MMM thats hinged in the middle. At either end of the bracket are double acting adjustable springs that act against each other to keep the bracket level, hense keeping the MMM level. Even when one side of the tractor dips down lower then the other.
I have the 60" MMM and am very happy with it's finish cut. I have read several owners of the 54" MMM raveing about the quality of their cut also. I don't recall any negatives on either once they are set up correctly.
I picked the 60" over the 54" for two reasons, 1 I wanted to get as alarge a cut on each pass as I could and 2 the 60" MMM pushes the BX out alittle further from whatever your cutting around which helps in clearing over head obstructions such as tree limbs etc.
Due to what I've read I'm sure I woul dhave been just as happy with the finish cut of a 54" as I am with the 60".
Volfandt