While Magic Arrow is a good skill in either tree, it is vitally important to Early-Midgame Thaumaturgy, but not particularly important to Early-Midgame Sorcery.
Currently, the first set of Thaumaturgy upgrades are "Enemy spells cost +4 Mana, Learn all 1st Tier Spells, and Heroic Strike does +10 Damage."
Sorcery likewise gets Magic Arrow as well as "All Unit Cooldowns are reduced by 1 (This isn't a real skill, as FP, not cooldowns are what gates ability use)" and "+10% Magic Damage Dealt".
Switching Magic Arrow and potentially also Cooldown Reduction for Thaumaturgy's Heroic Strike Damage and potentially Enemy Spells Cost More Mana would further cement them as "Cast Many Spells" and "Do More Damage", instead of having the spam spell in Sorcery and a HS damage amp in Thaumaturgy.