35mm is "preferred" 98% of the time by people on this site, due to the more care needed in medium format. I have shot skiing with my medium format "film" camera on a multitude of occasions. Even many other sports as well. Have not had any issues whatsoever. Medium format is not limited to a waist-level finder... I have a waist viewfinder, along with two other ae viewfinders that you put to your eye. As long as you compose a shot with a waist-level on a tripod for skiing, you're golden.
There is no such thing as a 35mm digital camera, unless you're referring to a cinema video camera.
My 1ds mk3 doesn't even come close to my medium format film, and it's 21MP. You are correct that medium format digital is amazing, although it's no different to use than any other digital slr.