it completely depends on what you want to do.
Umbreon is probably better for the more Tactical type of gameplay, such as using mean look, then baton pass, so you can switch out to a pokémon who has an advantage over your opponent, and your opponent cannot escape.
Espeon is a beast in Sp. Attack, you can use it to easily beat the elite four etc, once it gets a high enough level.
here you can find a complete analysis of any pokémon. some movesets however, require breeding which some people find difficult.
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you need to get evee and give it love in the morning to get Espeon and at night to get Umbreon
Make Eevee as happy as you can. For Umbreon, level it up at night. For Espeon, level Eevee up in the day.
Yes it is possible to get Espeon and Umbreon in Pokémon FireRed but you cannot physically evolve Eevee into either Espeon or Umbreon in Pokémon FireRed due to there being no time cycle in the game so you will need to evolve Eevee into Umbreon or Espeon in Pokémon Ruby, Sapphire or Emerald and then trade it over to FireRed.
There are eight. Vaporeon, Leafeon, Flareon, Jolteon, Umbreon, Espeon, Glaceon, and Sylveon
in the morning, after 8:00, you evolve eevee into espeon, at night, after 8:00, it evoles into umbreon (make sure eevee likes you alot)