I remember during the end of SoO, Varian says "We will End you," with Jaina pleading for Varian to slaughter them all; however, the Horde races/factions were split during this time period. Yet, that being said, Varian said it in a very confident manner, as if the Alliance "ending" the horde would be a relatively easy feat, even after the Horde was to regroup under a new leader (Voljin).
Thus, the impression that Blizzard has given, over the past few expansions, is that the strength of the Alliance is at least one order of magnitude greater than the horde's (if some numerical metric were to be assigned to either faction's strength).