The quick answer is that the XC is designed more for groomed trails and the RMK is designed more for deep snow/powder.
Here is a link with the spec's on one of your new sleds and if you click on 'specifications' on the left hand side of the link you can pretty much view any Polaris sled from any year.
http://www.polarisman.com/site/DisplayModel.cfm?ModelID=00SNO600RMK&Year=2000
Enjoy the new sleds.......