I love all the Eons, but between these two...Well, I really can't decide.
Espeon looks so elegant and mysterious and its Speed and Sp. Attack make it a good non-Legendary sub for Mewtwo. Arm it with Psychic, Shadow Ball, and whatever else you want and you've got a Psychic, Poison, and Fighting killing machine.
Umbreon looks badass and it's Shiny is, IMO, the best of all Shinies. It can definitely outlast Espeon in endurance, but its moveset and Offensive stats are to weak to put it to great use.