If you have sigiliph then get umbreon but otherwise espeon hands down. When you get eevee, which is very hard, make sure you evolve it into espeon so you can get psybeam. Additionally, a lot of hidden grottos might have eevee in them with its dream world ability(fennel will also give you one with its dream world at the end of the game). If you evolve one of these into espeon, it will have magic bounce, which is just awesome. Espeon will outspeed just about everything you meet for a long time.
I love both umbreon and espeon and if you want to you can get both.