I believe that choosing a faction depends on two things: 1) Who is your enemy...? 2) Your play style... If the enemy is Stronghold, then darkness is definitely the way to go; the light option only works in one case (yes, by the way, it's another way to counter Orcs) if you are Haven, and your hero is (don't be surprised)) Myv... If the enemy is Necropolis, then light is the way to go; if the enemy is Fortress, then darkness is definitely the way to go! (+readiness + battle griffons + vampirism)) But in general, it all depends only on you; there is no definitive answer here... :)