Medusa takes up 18 pop, Erinys takes up 55 pop.
1210/55= 22 ranged attack per unit, making Erinys slightly less effective then slingers.
425/18= 23.6 sharp attack per unit, giving Medusa a nice edge over hoplites, who have a sharp attack of 16.
However, Manticores can fly and have a sharp attack of 21 per pop, making them my favored attacking unit.
Cerberus, Pegasus, Centaurs and Cyclops make up the best defensive units, with a notable reference to Medusa who is decent at everything and great when paired with swordsmen, but not one of my favorites.
Cerberus and Cyclops specialty are both defending against distance. Cerbs offer a 52.5 defense against slings, making them more effective over Cyclops against distance weapons. However Cyclops make up with there smaller amount of sharp defense with a slightly better sharp defense, and since they're still better than swords with 30 some distance defense (too lazy to check exactly, I know the rest of these off the top of my head because I've done the calculations before) I like to pair them with hoplites. Hoplites provide the blunt defense, Cyclops give a slightly better distance defense then swords and a slightly lesser defense against sharps then archers, giving me a nicely balanced defense. However, like I said in the beginning, cerbs have almost 20 more defense against distance, which is why so many people prefer them.
Centaurs are a rarely used defensive unit for some reason. They have a 37.5 blunt defense per pop, making them the best blunt defenders in the game (beating out Medusa's 34.77 blunt defense). More importantly, Pegasus' weak spot is blunt defense. By pairing Pegasus and Centaurs together, with slightly less pop spent on centaurs, and then pairing that combination with swordsmen, you get one of the highest, if not the highest (haven't done all the math quite yet) ratio per pop defenses in the game.
The worst myth unit in the game: Minotaurs. With a 14 per pop blunt attack they're less effective attacking then horsemen, not great defenders and overall not worth building.