Umbreon looks cooler (In my opinion). Also Its a dark type so its super effective against Espeon.
My favorite Eeveelution.
espeon has magic bounce
Umbreon is more of a defensive pokemon, as it has a really high special defense. It's speed is fairly low, though, which I don't really like. Espeon has a high special attack, but low defense, and less HP. I think it's more of a preference, and more of what you need on your team, but I generally will choose Espeon if I choose to evolve Eevee into one of those two.