Lugia: Sky Attack > Future Sight?

I'd say both moves are on par with each other, just with slightly different applications, basically dependant on what you are fighting is weak or resistant to. Performance is similar, and can just boil down to the old 2 bar vs. 1 bar debate.

With Sky Attack, Lugia is viable as a Psychic counter, particularly useful if they have Focus Blast, e.g. Mewtwo, but you may already have something that helps in this regard, e.g. a Gyarados with dark moves.

In your case, I don't think you can go wrong with Future Sight.

I wouldn't waste a charge TM unless you have tons. My 96% Lugia has Dragon Tail / Sky Attack and I am saving him for Mewtwo. I like the 2-bar SA, but often would prefer FS for raw damage. But I'm not willing to waste a charge TM on him at the moment.

Because i don't think you would use a Lugia against Gengar or another Poison type, either move is viable. I personally prefer Sky Attack for the added option of Neutralling other Psychic-Types (Such as Mewtwo, as Lugia double resists Focus Blast variants and resists quick moves)

However, Future Sight does make dodging a bit easier since you're using charge moves less often.

http://www.pogomoves.com/check.php?lang=en&pkmn=Lugia&basic=Extrasensory&charge=Sky+Attack
Extrasensory/Sky Attack is the "best" moveset with marginally higher weave damage over Futuresight.

Both are super effective against fighting.

E/Sky Attack is a fast TM away from its best moveset if you want to use it as an anchor against Mewtwo with Focus Blast or Psychic.

Sky Attack provides the advantages of a two-bar move for sweeping a low motivation gym, but is resisted by rock which might sometimes be a small concern.

I haven't powered up a Lugia yet, but if/when I do, I would want Sky Attack.

I only have a 93% Lugia and it is level 30 with 150 candies left. I won’t even bother to power it up. Not worthy of my stardust at all. It is just too slow and only can attack like a xatu. How I wish I get all those rare candies back and give them to other useful legendaries.