Not 100% sure as I don't have the hard stats but this is how I've noticed it.
Mountain Dragon - Overheats really fast but does massive damage
Zephyr Dragon - Overheats slowly but doesn't do much damage
Sabre Dragon - Middle ground
As for ratio the Mountain Dragon does seem to have a worse ratio but it makes up for it by inflicting a ton of burst damage. As a result while it might overheat much quicker and might have a worse ratio it more than makes up for it by being able to inflict massive damage quickly while say the Zephyr Dragon might have a better ratio but it takes forever to really hurt someone making it harder to react with force using your dragon.
Each dragon is meant for a different role. The Mountain dragon favors burst damage to obliterate enemies quickly. The Zephyr Dragon tend to focus more on a support role buffing/healing units while debuffing enemies. The Sabre Does a bit of both.
