Fun Fact: Eevee-lutions love to cosplay as each other.

There's some odd similarities/matchups between them, like Sylveon's ribbons being somewhat akin to the locks than hang from Glaceon's crown, or both having pointy "socks", with Glaceon's pointing upwards while Sylveon's do so downwards.
They are also the only two with an advantage over Dragons.

I adore Glaceon, and Sylveon became my second favorite the moment it was announced!