The high end ones are more similar than different. It boils down to dealer support and price in many cases. I have seen the high cut quality of the Exmark, but it seems a bit less heavily made than some others. John Deere, Kubota, Gravely, Scag, Grasshopper, Dixie Chopper all come to my mind. In my area, John Deere support from my dealer (about two hours away) is very strong, so Green is the choice. Where my parents have their mountain place, John Deere is less strong so Kubota is the main player. I personally have had two brands, Gravely and John Deere and was pleased with both. The Gravely has a floating front axle and hydraulic lift, both rare features in this market. It cut fast but not that well. I know that they have since changed the cutting system and now they cut better. A heavier, more durable machine could not be found, though. I recently took order (last summer) of a 950 JD ZTR. It is a great machine. Super fast with a smooth cut. With the upgrades between the 800 series and the 900 series for 2010, John Deere would be at the top of my list, though one could not go wrong with any from the list I (and others) have mentioned.
John M