I have only used the bb720 mower from woods, but if the 8 and 10ft version are any thing like the 7ft they are definitely worth considering. I have used the bb720 just over five years. So far I have had to purchase new blades and replace the bearings on the right side rear tail wheel. It has, by my standards, been a solid mower. Definitely worth taking a look at.
I ordered the 5100M having never seen one. The 5320 I have been using has been absolutely great. I have had zero problems with it. When I started looking for a bit of larger tractor my first thought was the 5085E. But after reading and researching I decided that a 100 hp would be better suited for pulling a 15 ft cutter.
Another reason for going with the M series was the increased hydraulic flow. I use a 3pt bush hog backhoe from time to time to dig up stumps so i guessed the increased flow might help. I had even thought about adding a small hydraulic powered mower mounted to the front end loader in the future. I am not sure that would work though.