So, which one do you prefer?
Espeon, because I trained one once and it was pretty good.
Which one looks better?
Espeon. I like the mystic look it has.
Which one is more useful?
Depends. Espeon is good as a sweeper, while Umbreon is a great wall.
Which one would win in a fight?
Umbreon, because it resists Espeon's STAB. Maybe an Espeon with Wish, Signal Beam, Fake Tears and Charm could win though. It varies depending on the moveset.
I voted for Espeon, because I think it has one of the best design of every Pokémon.