But relating to the game, the Wolf as a symbol for the Horde has nothing to do with it's prowess as a predator, neither does the Lion for the Alliance.
As a symbol, the Lion stands tall and proud, and the rest of the animals in their habitat have a healthy respect for them. Lions are a symbol of pride and reverence.
As a symbol, the Wolf has fierce and unchallenged loyalty, and this is what the Horde is all about. While the Alliance is mostly united against the Horde, the Horde is united because of a since of brotherhood and loyalty amongst its main races, though other races have since joined out of convenience.
Symbolically, the Lion and the Wolf represent very different things, and to argue which faction is better based off of which faction's symbolic animal would beat the other in a fight is ridiculous.