All in all the elements play out like this: Acid is the best choice for utilizing the damage prevention ability as going through deck 4 it's the most common source of masterable element damage, and unlike fire a lot of that damage is going to get through because the conditional sources are hard for Seoni to avoid without blessings or other cards. It's poor for utilizing the +4 to checks power feat as there's only 1 spell that can inflict acid damage, and is the second most common element that monsters are immune to.
Where as acid is mostly defensive, Cold is the opposite. Through deck 4 it's the least common immunity, and most common weakness, and it has the most spells (tied with fire) that have cold as an inherent or optional trait.
Fire is a balance between Acid and Cold. It's the 2nd most common source of deck 4 damage after acid, but much less threatening overall because the majority of those damage sources are easy for Seoni to avoid. Offensively there are 8 arcane spells that can have the fire trait, though it's a somewhat common immunity possessed by henchmen.
Electricity is clearly the worst of the bunch. Not only is it a rare source of damage, a whopping 37 monsters are immune to it. Which is just as well, since only 3 spells can get the electricity trait.
Overall I think the best element to master first is acid. Because you get the damage prevention power immediately upon getting your role card, acid is the best out of the gate because it's the most common source of damage. Cold is the best for offense, but that doesn't come online until after the 4th scenario in deck 4. Once you get your first power in deck 5 the biggest benefit of cold - there are 8 spells which do cold damage - is lessened because you'll be able to add your mastered element to all your checks.
I'll update this thread as decks 5 and 6 are released, as the contents may alter the balance between the elements. Likely electricity will still suck, though.