i completed both campaigns, and i found evil to be more fun. haven't tried multiplayer so perhaps that adds a lot more playability, but once you've beaten the single player, you've beaten it... there's not really a want to go back and do it again if you conquered every province the first time around.
also, about repetitiveness, which i'd assume is a campaign issue and less apparent in multiplayer: i find that a lot of the missions are similar, involving massing a huge army, hotkeying my hero to 1, army to 2, and obliterating the map until i reach a castle. then the game practically stops while my army stands around waiting for seige units to build and report in, before i crush whatever is left alive. the defense missions were much more strategic, as was the one battle i entered with the "wrong" army by mistake, taking a 10 orc force into an immediate battle that took a lot of work to fend off attack squads while forming my army. unfortunately for the most part these were rare moments of difficulty in a really easy (strategically) game.
also, at least in campaign mode, some units were much overpowered. ranked and upgraded archers in large numbers were the apocalypse incarnate upon every force i encountered except for castle walls, which required me to build seige rams and ladders.