Physically, tauren will always be stronger, however given there size and strength, I can't really picture a tauren as being a character with finesse. An orc or human could probably outmanoeuvre a tauren if they were fast enough. after all based off of lore and game mechanics, orcs and humans can be both physically strong and yet be subtle enough to be rogues.
So tauren are stronger, but orcs and human have a bit more balance of both strength and finesse.