Higher damage =faster DPS. If they both cost 10mp, then rank 1 is better.
OP mentioned Ice Wyverns. It's safe to assume that Firebolt is being ranked only for ballooning, in which case DPS doesn't matter if every hit is OHKO. The lower MP cost of Rank 2 compared to Rank 1 will save a large amount of MP pots if you can get the amount of stars you want before running out of MP, since you can refill in between each balloon.