Yes, there is a big difference, although the "horsehide" White's uses is incredibly nice and has more character than shell cordovan. True shell cordovan is only produced in the USA by Horween, and is from the butt hide of a horse. The special, complex tanning process produces comparatively small "shells" of hide that are very dense and have special properties that make them ideal in certain applications, such as some boots and shoes. Here is a video where you can learn much more about this remarkable (and expensive) leather. http://vimeo.com/4814754
A pair of White's semi-dress boots made from shell cordovan cost about twice as much as a pair of SDs in the "horsehide". When I was growing up, cordovan was just a little more expensive than calf hide and lasted much longer, so my parents always bought me cordovan dress shoes to SAVE MONEY because I couldn't wear them out as quickly!