Sylveon: Sylveon is one of the best clerics in competitive, but not in in-game. Thanks to its Fairy typing, it can kill powerful Dragon, Dark, and Fighting type Pokemon. Also, It has a good special attack and special defense, but it has a low speed of 65. It also has access to Hyper Voice, Dazzling Gleam, and Heal Bell, which allow it to hit through substitute, both opponents, and get rid of damaging status conditions. Additionally, it can take most special attacks. However, some physical attackers and Poison-Steel type like Scizor and Tentacruel can KO Sylveon quickly. And his speed stat are not good.
Espeon: Espeon has a fantastic ability named Magic Bounce, which allows it to send status attacks such as Toxic and Roar back to the opponent's team, but unfortunately, It's hard to get a HA Pokemon in in-game, but Synchronize also works great. It can pass status conditions like burn and poison to its foe. Espeon has a super Special Attack and Speed of 130 and 110. Psychic, Dazzling Gleam, Shadow ball are awesome special moves with Espeon. While Espeon has a decent Special Defense that allows it to take a few special attacks, especially alongside Morning Sun. Similarly to Sylveon, who has weaknesses to Steel and Poison type moves, Espeon also has two weaknesses, being weak to Dark- and Ghost-type moves. But it has pretty low HP and Defense that makes Espeon easy to kill.
Both are good, but I would prefer Espeon.