For a slight increase in spellpower, use Spell Wisp, then use Spell Might. Spell Might will grant slightly more spellpower than normal because the mage has a greater spellpower score after using Spell Wisp. Even if you deactivate Spell Wisp, Spell Might will grant as much spellpower as it did when turned on. If you then turn on Spell Wisp again, the spell will grant you more spellpower for the same reason Spell Might does - your spellpower is greater when you activated the ability. So in short, to make the most of the two spells, activate Spell Wisp, then Spell Might, then deactivate and reactivate Spell Wisp.
The spellpower bonuses are determined when the mage casts the spell; changing equipment while the spell is active does not change the spellpower bonuses.