Functionally they are the same. Whitworths, especially the 375's & .458's typically had a nicer grade & shaped stock, better finish, and nicer sights.
MkX's can often be over polished (wavey exterior). Whitworths tend not to be so. There are cheaper grades of Whitworths though that to me appear no different than their Mk X counterparts and sport the same "functional" stock.
My own .375 H&H ws built on a .375 length Mk X action with a Lothar Walther barrel. It functions just as well as any .375 Whitworth that has come through my shop.
