what Lugia shuld i power up?

Don't listen to this guy. I would lean towards the 15 Attack because the IV difference isn't that huge. With Pokemon like Lugia, Attack isn't always enough to ignore IVs completely. However, there's only a 2 IV difference.

Meh. If it is rare and not easily obtainable (like Mewtwo) or no longer available (like previous legends) then IV snobbery should be a bit more flexible.

And btw 40/45 IVs is 88.8888% so depending on how an IV calculator rounds it can be displayed as either 88% or 89%. As long as you are consistent so you can quickly compare different IV totals of the same species, it doesn't really matter.

You can use a 86 ivs Pokémon, a 91, 100 or whatever ivs for the same purpose, ivs like 30, 40 or 50 is awful and it won’t reach decent cp according to its species, but at 80’s ivs (preferably 82+) and with 14 or 15 attack ivs the Pokemon is worthy

Your standards are really really high, I guess. My best Lugia is only 80% but I'm still gonna power it up soon.

May I know what is so special about >90? It's not like the tiny difference in survivability will make it or break it in a battle, when Lugia is already as tanky as Snorlax.