For farm and ranch work, I would not recommend the Rhino. Even though I own and drive a Rhino, and do a fair amount of work with it, the Rhino really wasn't designed as a work vehicle. But, for extreme terraine, reaching those hard to get to hunting spots, the Rhino is a very good machine. Also check out the 2008 model Kawasaki Terryx, which sports the 750cc V-Twin.
For work, looking only at the vehicles you listed, I would probably recommend the Ranger (must use low range for hauling/towing or risk ruining the belt on the 500cc model) or the new Gator. The newer Gator model HPX I think is what they are calling it, seems like a winner. For heavy duty work and proven dependability, I would recommend the Kawasaki Mule 3000 series, preferably the 4WD model, 3010. Upgrade the tires and add a winch and you should be happy for many years.