Order
The Order has little synergy with the Amurites. Unless your mages are marching on the world and matinence is crippling, or the AV is going crazy, it's rather pointless.
Empyrean
Since mages are often weak and assasins are always targetting them, Empyrean is good. Ratha to kill assasins, along with Blinding Light to stop enemy units from killing your mages is useful. Also, Chalid is awesome.
Runes of Kilmorph
Excellent for teching to Sorcery, afterwhich, changing to other religions might be useful.
Fellowship of Leaves
Lorewise I like this because FoL is an opposition to winter. I also love their priests, though there is no unique synergy.
Octopus Overlords
Hemah is a great mage, and the Asylum provides useful sages.
Council of Esus
Great for taking out enemy mages and Gibbon.
Ashen Veil
Good for sages, but the Hell terrain isn't worth it IMO.
Overall I'd go for CoE or Empyrean.