I'm pretty sure it makes any light wizards miscast on doubles, in addition to them picking any lore in the game at the start of the battle and making it -2 (3?) to cast for the whole game. Very nasty banner.
Anyway, yeah I would be changing lores depending on your opponent. If you have to pick 1 and stick with it, metal is a little bit more of an all round lore than light, light is a little bit limited. Though as an all round lore I would probably go for death, beast or fire instead of either of them.