The Rhino and the Grizzly ARE NOT BUILT ON THE SAME FRAME AND CHASSIS. The chassis resembles the Grizzly but is longer, and wider. That being said, there are many similarities like using the same engine, transmission, front and rear differential and locking 4WD.
I bought the Rhino 660 last year and love mine. We use ours for hunting, fishing, and general riding around / exploring. After trying both the ranger and the rhino, it was the comfort of the bucket seats in the rhino vs. the bench seat in the ranger that sold me on it.
I haven�t had any problems with my 4WD, and I use it a lot on tundra, beach, swamp, and steep inclines/declines on rocky terrain. The Ultramatic transmission has also been great and has super smooth shifting. I�ve also done a bunch of creek/river crossings with no slipping.
One thing I can say is for what I do the stock tires aren�t going to cut it. Great on hard surfaces, but when I get in the muck, they suck. The stock tires on the Ranger (rawhides I think) look way better. I�m going to change my tires out this year. You should also factor in the cost of a good winch.
If you�re looking for a dedicated hunting rig, I�d take a serious look at the ranger. Lots of power and size, and huge carrying capacity. When I was hunting last fall there were times I wished I had the Ranger 6x6. The Rhino was very capable and got the job done, but there may have been a better tool for the job.
Try as many as you can before you buy and find out what�s right for you. The quality of the polaris and the yamaha seem to be comparable.