From a strictly power level standpoint, I can't justify running Elvish Mystic. It's only one elf! Llanowar Elves and Fyndhorn Elves are both elves! Two elves for one mana! Ambiguity!
From a flavor standpoint, Llanowar is a place where you'll have a bajillion broken bones. The guys from there? I don't want to piss them off, they'll be on my team. Fyndhorn looks like a great place to get shanked. The guys from there? I don't want to piss them off, they'll be on my team. Elvish Mystic, the "new mana elf" is led by Nissa, the Elf Planeswalker who ... can't produce mana. Yeah, we know who's getting picked last for the dodgeball team at Elf summer camp. If only Elvish Mystic had just followed Garruk, it would probably be a strictly-better mashup of Arbor Elf and Ulvenwald Tracker. Alas.