Like the box blade, the work and tractor determine sizing/grade. Your plan entails venturing into a lot of unknown territory initially, which is going to beat up any cutter. The duty rating tells you what kind of a beating the manufacturer is expecting the unit to take as well as how big a machine you can put it on without overpowering the transmission. For a field you don't need thick sidewalls and 150 hp gearbox, but for pushing against saplings, smacking rocks, ledges, and stumps, you need to be able to handle the shock. All things equal, you'd be better off with a heavy beater until your clearing is done... once you know where all your danger areas are, and you're in a mowing routine, you won't need the driveline and construction to be so robust, and can have a shiny one with undamaged blades, sidewalls and deck to last a lifetime mowing the field. It won't last clearing parts unknown, but thats part of the deal if thats your application.
Definitely get a slip clutch on any unit you decide on--you need driveline protection for your tractor's transmission.