I'm surprised no one made this one yet.
Espeon and Umbreon, the original pair of light and darkness. Both are Eeveelutions intoduced in the second gen. And, interestingly, they both recived updates to their officail artwork with the release of FireRed and LeafGreen, as opposed to most Johto pokemon who didn't recive artwork updates until HeartGold and SoulSilver.
However, this is where similarities come to an end. Espeon is a fast, powerful, and frail special sweeper, while Umbreon is a wall, primarily on the special side, and has little in the way of offensive power or speed. Umbreon fills a unique place in standard, having access to moves such as payback and toxic, while Espeon is outclassed by Alakazam in almost every way and is only seen in NU.
So, which one do you prefer?
Which one looks better?
Which one is more useful?
Which one would win in a fight?
Personally I have to go with Umbreon all the way on this one. It I always feel like Umbreon is the unappreciated twin who's overshadowed all the time.
As for looks, that one's close, but I have to give it to Umbreon.
Umbreon is definitely more useful.
Umbreon would win. High special defense + payback = Espeon fainted.