S (Shield)

Strategy and Synergy
There are plenty of spells eligible for this upgrade, but S (Shield) finds more use on defensive or fire-and-forget spells, granting a small bump to shields whenever they are cast.

A pretty straightforward upgrade, but there are better options around, specially with shield-granting spells, as those are eligible for the superior upgrade. If the other two upgrade options aren't worth considering, taking S (Shield) may be your safest bet as it won't disrupt your pacing neither the deck all that much, but will increase the requirement by 1 for subsequent upgrades on the same spell.