Neither.
What I'm about to jot down is the best Sylveon set out there, and it's for a reason:
Sylveon @ Choice Specs
Ability: Pixilate
EVs: 240 HP / 252 SpA / 16 Spe
Modest Nature
- Hyper Voice
- Psyshock
- Hidden Power Fire
- Baton Pass
At least in OU that is. Now, OU has been infested by steel types ever since GSC first introduced them, and has only gotten more popular since. Creepers like Ferrothorn, Scizor and Skarmory are still very viable, and Shadow Ball isn't getting you too far. Dig is a very bad option as it comes through it's measly Attack stat. Your best option here, is Hidden Power Fire.
Or, if you are afraid one fine day a Heatran will mow you down, you could also consider Hidden Power Ground as a solid option.
If neither is available (due to needing specific IVs for Hidden Power) then go for Shadow Ball.
Hope I helped!
