Sky Attack is better in matchups with equal effectiveness (e.g. neutral effectiveness or vs Machamp where both are super effective) but Future Sight has it's uses as a pure psychic type attacker.
If you want to use it for soloing Machamps and/or clearing gyms with less potion usage, you'd want Sky Attack due to better neutral DPS.
As a psychic type attacker in raids it can be useful as a tank counter if you have issues rejoining after your 6 Pokemon have fainted or you just want to save on revives.
Otherwise there are better options for psychic types if you want more bonus balls for damage, such as Mewtwo if you're fortunate, or glass cannons like Espeon or Alakazam, while Gardevoir and Metagross are compromises between glass cannons and the super tanky Lugia, having only slightly lower attack but much better bulk.
Personally I'd go with Sky Attack. As a pure psychic attacker it's outclassed or on-par with several Pokemon, but with Sky Attack it's one of the best counters to Machamp and #1 for potion efficiency when fighting gyms, so I'd be more likely to use it.
