For pure casters (Avril, Cleo, Vallari), would Zen Cauldron or Hand of Wisdom be best? From first glance, you would say Zen Cauldron would be better as it has about 10 more magic vs the Hand of Wisdom. But I really question whether it's better than the Hand of Wisdom as in a typical fight, only one offensive spell is cast per fight. Avril's Blizzard damage is terrible and is really only useful to lock down troops. Cleo and Vallari could certainly benefit from the magic but would +20 magic really out value the stats that two Hand of Wisdoms provide for army bonuses? I can't imagine that +20 magic for one offensive spell out values 12 command (army attack speed) and 18 stamina (army health).
Hand of Wisdom
+4 Magic (Reduces magic damage taken by hero, who cares..?)
+11 Command (+Troop Attack Speed)
+9 Stamina (+Troop Health)