Arcane trait is actually weak in my option, it doesn't give any economy bonuses, and it doesn't have any MAJOR combat advantages, and having cheaper mages guilds feels alittle pointless if you get the wonder that gives a free mage guild in every city.
Divine casters are much easier to make, seem to gain exp faster, and I haven't played in shadow yet so I don't know about the guild or if there been any major changes to the altar, it comes with more advantages then +3 production +1 gold priests, like happiness in cities, which is a huge bonus, extra experience, can get some strong units right off the bat, and it allows you to assign more priest specialists to the city, which is good early on.
I usually built it just for the +2 production to priests, which combined with priesthood and agriculture, let's you have a huge amount of production in every city.