Unit cost and quality.

From personal experience, I've found that the Ifrit is actually better. Strangely enough, its combat tactics are similar to those of a Griffin. The Ifrit also flies into the enemy ranks and "treats" everyone who attacks it. As for the Nightmare, unfortunately, after using its spell, it turns into cannon fodder. However, the Ifrit is vulnerable to direct damage spells and quickly turns to ash. But magical attacks affect the Nightmare much better. Although combat mage units are encountered much less often than archers. So, my choice is the Ifrit.