Espeon/Umbreon?

Vaporeon.

The most useful aspect of perfect IV Pokémon imo is the high max CP. Therefore I'd take espeon, which actually has relevant max CP at about 3000. Umbreon only has about 2000 and 20 up or down doesn't really matter.
Espeon you might place in gyms and there it matters

Espeon. Niantic's CP formula or CP centric gameplay makes it no sense to go for lower CP Pokémon, unless you're really desperate for variety or just need to fill your Pokédex.

Espeon for high max cp and better gym placement.

Umbreon is a good prestiger, which only requires a cp of 1,000-something. Pretty much any high level Eevee could be a good prestiging Umbreon regardless of its IVs.

Hey, I'm in the same spot, but I already decided what I want to do. I'm going to wait. I want to be sure about two things: which one of them is better (Espeon seems to be better, but by how much?) and on which one is it easier to get a good moveset. If Espeon is only better by a bit, but it is easier to get a good moveset on Umbreon, I might still go for Umbreon. Obviously you can analyze their movesets already, but I want to wait until Gamepress rates them, because I'm not that much of an expert.

Espeon, is the higher CP one... if you really need a Dark Type and you don't have acces to tyranitar (which I don't think it's that good as a Dark attacker) or Houndoom, then go for Umbreon, but I think you'd be better off with a good houndoom.