As far as I know both Articuno and Lugia are not (much) better damagedealers than the best DPS Pokémon in their class. I am talking about purely damagedealing. Maybe they can take more damage and in the end they are a better counter. But I am not convinced yet.
Both Alakazam and Espeon are better (Glasscanons) against for example a Gengar or Machamp raidboss.
And a good Cloyster/Lapras is as good as Articuno.
So I am not going to spend any stardust on them (yet). Or not too much.
Although I have a 100% IV Articuno and it's really tempting to max it out.