I'll have to say that I much prefer Espeon to Umbreon. I never really did like the idea of "Dark" Pok?mon. . . . In my opinion, Espeon is more graceful and sophisticated--Umbreon is still graceful, don't get me wrong, but I've just always been a fan of the more sophisticated and elegant style Pok?mon (hence, Ninetales in my username and avatar).
Though in battle, I find them both to be useful--I use them both in Silver Version. It seems like they are both opposites on everything: the time that they change from Eevee into one or the other (incidentally, I nicknamed my Espeon "Sunrise" and my Umbreon "Midnight"), their types, and their battle techniques. Espeon is strong on offense, while Umbreon is strong on defense. Interestingly enough, they both have their own healing moves (Espeon has Morning Sun, Umbreon has Moonlight). Espeon can finish a battle quickly and sometimes make it through without a scratch, while Umbreon can stay in battle and finish everything slowly, still coming out with a lot of HP. It really just depends on your preference--what you think looks best, and what your battle style is (if you play the video game, that is).
