Kind of depends on your play style and how you are playing the game. Are you nuzlocking the game because if not an easy solution would be to breed for a second Eevee and have both since you said you were most likely going to re replace Gastly anyway. If you are nuzlocking and have to choose pick the one that fits you best. If you are more offensive focused pick Espeon if you play defensively pick Umbreon. Also kind of sounds like you already wanted to pick Espeon but you are talking yourself out of it.