To sum it up, it has better speed and has priority. They also have the same attack stat, so having higher speed is better. Dusk form is obviously serperior to both, but It's hard to obtain now.
For your eeveelution, You already have fairy, water and electric or fire. From this, I'd say leafeon, because it's HA is good, you can cover its fire and flying weaknesses, and it's attack, defense and speed are great.