100% Eevee

Sakura...even if its a male.

Espeon will benefit more from high IV's while any old level 20 eevee can become a decent Umbreon prestiger.

Because Espeon has high enough max CP to place well as a defender, it will get more value out of 100% IVs.

Umbreon even with 100% IVs will unfortunately not have high enough CP to be worth placing in gyms.

Umbreon is pretty disappointing in PoGo, much like Wobbuffet his strength is not about brute strength

This alternative might be way less satisfying with potentially no payoff, but I'm just going to throw it out there because it's been on my mind lately - especially with spring around the corner...

If it was mine and it was a female Eevee, I'd keep it for potential future breeding. If you've found 100% Eevees to be rare like I have (caught 2, hatched 0), consider that on average you'll only get 1 female for every 7 male.

It might be a while before we know if/how breeding might be implemented, but at my rate of getting 100% Eevee's, a female one will come around every 2 years on average.

If it's male, I'd say go with your gut and the advice here so far: Espeon.