I prefer elementalism because it works well combined with other parts of the army:
Plague of Rust for the -2 AS that works on shooting AND can be cast on a unit in CC; cast it on an advancing unit; shoot stuff at it and then cast it again when it gets into combat.
Earthen Ramparts works well on a unit of Saurus Warriors or Temple Guard with their stats. I like marching one turn and then advancing with this spell cast on them. They are slow regardless so they are hard to get into the CC of your choice. And if your opponent wants the points they are going to have to charge it anyway.
Important note: both these spells last until next Start of Turn sub-phase, so during both your turn AND the opponents turn.
Wind Blast is one of our (imo) best counters vs heavy cavalry and especially if you can combine this with Plague of Rust. Chuck this through an Arcane Vassal to remove the range issue.
Flaming Sword is pretty damn bad, regardless if its an Assailment or not so I would never pick this spell. Summon Elemental Spirit is OK but imo our Monsoon is better (larger template, larger blocking of LOS and sometimes you might get lucky with Drenched); Travel Mystical Pathway seems OK but I am not sure how well it would work. Signature Spell isn't horrible but imo isn't going to impact us as much because we already have a low I and M. Can't be cast into CC either
Keep in mind I run my Slann with Lore Companion and for now I will be picking Plague of Rust, Earthen Ramparts, Wind Blast and Monsoon most of the time.