I actually have all three. A Kubota BX (26hp) with a mid deck, A Kubota B3000 (30hp) with a mid deck that I also run a six foot finish mower behind. The mid deck does get in the way a bit for doing other tractor stuff but I just leave mine on all the time and it is not that bad. I run a disk, the finish mower, a sprayer and box blade work with it on. I like both the finish mower and the belly mount and either will work. The advantage of the belly mount is that it is easier to get close to objects, easier to judge, than the finish mower behind the tractor. If you have a lot of obstacles to maneuver around then you will like the belly mower better. The finish mower does tend to give a better cut since the mower is riding on it's own wheels as opposed to being under slung from the tractor like the BX is. The B3000 is the best of both worlds since it is a belly mower but also a finish mower style since it runs on it's own wheels for depth of cut. I know these are all Kubota's as I have used for examples but I am also an RK guy, but I have the RK55 with grapple and I run the finish mower behind it as well.
I have the RK-24 with a 60" three point finish mower. Like Pecan Farming said, their good for open areas where you don't have a lot to mow around. The cut is very nice, rated right up to (in my opinion) a zero turn and easy to remove when you want to use the tractor for other work. The RK-24 handles a 60" just fine, just keep your pto rpm's up to 540 and use low range.