I thought I would bump this to see if anyone has any more thoughts on the possible benefits of the two mowers for slopes. The two I am looking at are the Gizmow Formula: http://www.gizmow.com/home.htm and the Cub Cadet Z Force S: http://tinyurl.com/27fbugn.

Both are Zero Turn, but they do not have the loose caster front wheels typical on ZTRs. They have a steering wheel to control both the rear wheel drives and the front wheels. The Gizmow is promoted as more stable on slopes than normal ZTRs. Their video demo is pretty impressive. http://www.gizmow.com/video.htm. I believe Massey Ferguson has a couple of similar models.