I drove both and was going to buy the Ranger 6X6 to go with my RTV. However, two of my friends who have the Rangers talked me out of them. I've put this in earlier threads, but I'll mention it again. I have nothing out for Polaris, but maybe for them to improve their durability. Both friends who bought the Rangers had them in several times during the warranty period and several times after the warranty period. They both kept suffering from numerous problems.
The one guy tried to trade his Polaris on a Rhino, but the Yamaha dealer told him that he'd be better selling it outright. In other words, they wouldn't take his in on trade. He is a bit hard on his units, but so far he can't break his Rhino. I am not as hard on my Rhino, but I have added the BlackRhino CDI unit that allows it to run over 50, and several other adds.
After having it, I'm really glad I bought it. I don't see how you can beat one. My only wish is that it would be fuel injected. I'm not used to using the choke and warming it up in the cold weather. Other than that, it's a true beast. Do some searches on Google or any other ATV "side by side" forum, and you'll find the Rhino the one to beat.
When it's your money, you need to buy what you want, so try all you are interested in and read up on what other owners have to say about their units. Good luck! I wouldn't trade my RTV or Rhino for any other units. For me, that's saying something. If I don't like something, it's gone! Right now, I'm just waiting for the 27" Maxxis Bighorn Radials to come on for my Rhino. I have the 25' Bighorns on my RTV and I love them! The traction is tremendous and it actually improves the ride even though they are 6 ply tires.