I've driven both, bought the Rhino. Ranger is an excellent machine with more hauling capacity, more towing capacity, and one more "seat". It's faster stock at 700 CC EFI than the Rhino 660. A few things made up my mind.
More after-market stuff for the Rhino
Rhino is narrower than the Ranger, fits better in my hauler and on tight trails
The CVT/engine brake feature on the Rhino is great when moving through the dunes or over rough terrain, rarely have to touch the brakes
Fit and finish on the Rhino is, in my opinion, better than the Ranger. There is a lot of plastic on the Ranger, less on the Rhino.
Price difference was, in my case, about $1,500...that paid for my 4-seat cage and most of the back seat
I read a ton of forums and found that all side-by-sides have problems...most of the serious SXS posters recommend Rhino over the Ranger
I have 600 miles on the Rhino and we've driven everything from dunes to soft sandy trails, to rough, rocky trails, to open desert and even a dry lake bed. It's done everything I've asked of it and more.
I think the big deciding factor for you will be what you plan to use it for. Will it be for play only? Or will you be doing farm work too? If you're going to use it for work on a regular basis, you probably want the Ranger for the greater hauling/towing loads.
That's my humble opinion. Good luck!