Hugh,
I have a 72" Bush-Hog brand box blade that has handled all I could throw at it. I think it weighs about 500 lbs. The Rhino looks like a good one too but I don't have any experience with it. Any good HD unit will cost more but will be more difficult to tear up! I also have a Bush-Hog brand cutter, 286 model I think, with chains. It's a good medium duty cutter, as is Rhino. I would stay away from the B/H "Squealer" line or other light duty cutters if there is any tough cutting in your future. You generally get about what you pay for. Good luck.
PS: If you haven't bought a tractor yet consider top & tilt. If that's too expensive right now, at least be sure to get dual rear remotes as it will cost less now than to add later.