For most stock situations 35mm chromes are the way to go. Unless you are doing something very specialized. I shot stock for years and found 35mm sold just as well as MF. Now if I'm dealing with art directors directly, then MF is the way to go. They are impressed with the size over 35mm.
These days it seems most of the ad agencies want to see digital files so you might as well shoot a D1 or something like that.