I bought the rtv 500 a few years back and love it. I went shopping for a four wheeler and ended up with the 500. For me, the machine is for light work, not trail riding, and easy on the budget. There's times I'd appreciate the 900 but at the end of the day, between the tractor and 500, I get everything done. If you don't have a tractor and will be doing heavier work such as snow plowing, I'd be considering the 900.
Regarding the decision between kubota or polaris, if you prefer riding trails and handling at high speed the polaris will be better for you. If you will be working it hard you can't go wrong with a kubota. Also, research the difference between the transmissions of each machine you are considering. Kubota's transmission was a selling point for me.
One more thing, I recall an advertisement a while back for a new line of kubotas to compete with the trail machines, more ground clearance and better suspensions. I haven't researched them much but that might be an option too.
Good luck!