Personally, I'm not sure what level most people are at when they doing Bahamut's Cave... but I'll try to be fair here.
The random point for this discussion to look into how good or bad Leviathan is when you're using it vs the Behemoth battles. I used to use Leviathan for these battles, but at one point I simply just gave up, since the damage return on casting it was simply not MP efficient for me. I'm posting here to see if the damage is at least reasonably higher than Mist, ignoring MP consumption.
Leviathan has a base spell power of 280.
Mist's spell power is directly related to Rydia's current HP.
For the optimal situation, let's see how powerful Leviathan can be... based on a particular level range.
At level 42, Rydia has 40 Wisdom and an HP range of 1110-1245. At level 52, Rydia has 48 Wisdom and an HP range of 1670-1875.
Rydia's equipment is this set... Charm Rod, Tiara, Sorceror's Robe, and Rune Ring. This increases her Wisdom by 28.
Therefore Rydia's spell power at level 42 is 18, and at level 52 it is 20.
Ok, onto the math:
280 (base power) * 1 (worst case) - 254 (monster's magic defense) = 26 * 18 (multiplier @ level 42) = 468 (@ level 52 = 520)
280 (base power) * 1.25 (avg case) - 254 (monster's magic defense) = 96 * 18 (multiplier @ level 42) = 1728 (@ level 52 = 1920)
280 (base power) * 1.5 (best case) - 254 (monster's magic defense) = 166 * 18 (multiplier @ level 42) = 2988 (@ level 52 = 3320)
So, by comparison, Leviathan is stronger on average... on the other hand, it's hard to justify the 30 MP increase (or by 2.5x, however you factor that in.
At maximum Wisdom though..., the damage range is 650 - 4150, with an average case of 2400. By this point, Mist has greatly surpassed this.
So, unfortunately my best suggestion for Leviathan is to use it instead of Bahamut if you want to conserve 10 MP vs low magic defense targets.
5040 minimum damage vs a 0 magic defense target @ level 42... 7000 minimum damage @ maximum Wisdom. Bahamut is generally overkill for low magic defense targets.