Okay, here is your answer:
Skarmory has higher attack and defense.
Umbreon has higher Special attack and Special defense.
Then, Skarmory has higher speed.
Weaknesses and Resistances
Skarmory is weak to Electric and Fire types
Umbreon is weak to Bug and Fighting types.
Skarmory is resistant to: Bug, Grass, Psychic, Steel, Flying, Dragon, and Normal
Umbreon is resistant to: Ghost, Dark
Obviously, Skarmory is better because it has better stats than Umbreon, plus, it can cover weaknesses much better.
Hope I helped!