Ok I think I changed my mind after I checked both pages. greater storms has more dps, like 100 damage, 10 times every 6 seconds (when Frontier Spellbook isnt in eternal form.) Im not totally sure what the normal dps of the Frontier spellbook is but I predict its similar cause it does 140 dmg with a slower fire rate. However, the better special move is obviously Frontier spellbook. Kinda forgot that the frontier spellbook got that buff lol so I thought the other was better.
Basically in the end I didnt really check and I agree with you.