In an all offensive match the Alliance have the advantage. In a situation where both sides are mostly offensive with a few defenders for back capping its roughly even. If it turns into a purely defensive battle the Horde have a fairly large advantage.
The problem is though is that basically the Horde can almost always dictate the type of match there will be. Basically the Alliance can’t use their advantage unless the Horde allows it, meanwhile the Horde can always pick a match that is to their advantage.