I think familyman gave an excellent explination. The higher gpm on the hydraulic pumps will give you some quicker movement of your FEL, in practical terms to users, unless there is a large difference, you probably won't notice a difference of less than 1 gpm on most machines. So from the user standpoint 5.5gpm versus 6.1 is going to feel very similar, technically 6.1 is better.
As far as the cat 1 versus limited cat 1 hitches, this is a point of debate that has occured several times. Someone posted a while ago that thier JD sales rep told them the hitch on the JD2210 is a Limited Cat 1 hitch. I never confirmed that. The BX hitch is pretty well documented to be limited. Some people have swapped out the lower link arms with larger ones, others have drilled theirs to alter the geometry, many have modified their implements. To my knowledge, and I admit this is limited, in the SUB-CUT category the MF and the NH are the two that have full Cat 1 hitches. Or at least it seems to be that way. There are other small tractors that occasionally get lumped into the SUB-CUT class, but really are small frame CUTs like the Kioti CK20 (which is basically the same size as a Kubota
B7510 or NH TC21), at that size machine, all have Cat 1 hitches. Personally I would not buy a machine that had a limited Cat 1 hitch, but lots of other people do and are quite happy. You need to ask what implements you want to use and then let that help be a deciding factor. I swap implments between different machines, I need to have a Cat 1 hitch; others may not have that need.
In terms of reliability, understand anyone can buy a "lemon" of a machine from any brand, but that aside, I think you'll find that MF, NH, JD, and Kubota all make outstanding Sub-CUTs.