Gym Leaders, E4, and other notable opponents in XY.
Viola – You can't get an Eevee before her, but if you could, her Bug-types would threaten Espeon.
Grant – His Tyrunt is part Dragon-type, meaning that Sylveon's Fairy attacks can hit it supereffectively.
Korrina – Both Espeon and Sylveon can take on her Fighting-types.
Ramos – Espeon will be able to hit Weepinbell for supereffective damage.
Clemont – His Magneton resists both of Espeon and Sylveon's STABs, and can hurt Sylveon with its Steel typing.
Valerie – Mawile and Mr. Mime resist Psychic while Mawile resists Fairy and can threaten Sylveon with Steel attacks.
Olympia – All of her Pokémon resist Espeon's STAB.
Wulfric – Neither Sylveon or Espeon are better against him.
Wikstrom – His Steel-types can defeat Sylveon and resist Espeon's Psychic STABs, while Aegislash can easily take it out with Ghost attacks.
Malva – Her Fire-types resist Sylveon's Fairy attacks, but Chandelure can threaten Espeon.
Drasna – Sylveon wins against all of her Pokémon but Dragalge, who is hit supereffectively by Espeon.
Seibold – Starmie resists Espeon's STAB.
Diantha – Sylveon can take out Tyrantrum, Hawlucha and Goodra. Espeon is hurt by Gourgheist, and both Eeveelutions struggle against Mega Gardevoir.
Lysandre – He uses a Mega Gyarados, which is partially Dark-type, meaning that Sylveon has the advantage… but watch out for Iron Head.
In conclusion, I think that Sylveon has a better matchup in-game. However, both Sylveon and Espeon have the same coverage (Dazzling Gleam, Psychic, Shadow Ball), but Sylveon has easier-to-cover weaknesses.