I asked a similar questions a few weeks back and got mostly post that favored horsehide and a few for cow.
Horsehide: Thinner profile, and breaks-in faster. Some dispute over its ablity wear longer than cowhide.
Cowhide: Price (sometimes), appearance, availability, vegetable tanning process was designed for cow.
some quality leather shops (Rosen, Horse shoe) state no clear advantage of horsehide when compared to the best cowhide (A quality bends).
Try these sites for more info: