It's not relevant for Humans; both water and air spells are decent at low levels. But the skill level and high/low magic affinity make a noticeable difference. When it's not advantageous for demons to use fire, and it is advantageous for undead.

Guardian Angel is a Human special ability in Heroes of Might and Magic (although initially, Mentor was planned as the special ability). There can be no talk of any additional effects in combat (technically). But in Heroes of Might and Magic, the ability works stronger because there is much less "meat," effectively raising an entire stack. Plus, the hero also learns resurrection. And after each battle, losses are reduced by half (the idea was borrowed from the racial Human ability in Heroes of Might and Magic IV).