I've never seen a mage (let alone warrior) with a full card set use violet thunderstorm instead of snow sapphire; SS's effect is vastly superior and they are otherwise identical. This suggests that one should be nerfed, one should be buffed, or they should both be changed. How about a small nerf to SS's mitigation, combined with either a buff to VT's effect (option 1 below) or a buff to VT's casting cost and durability and a nerf to it's mitigation (option 2 below).

SS: cost 6, durability 4, mitigation 1 (instead of 2), effect unchanged

And

VT1: cost 6, durability 4, mitigation 2, effect = 2 damage (instead of 1)

Or

VT1: cost 4 (instead of 6), durability 5 (instead of 4), mitigation 1 (instead of 2), effect unchanged