Power up articuno or lapras?

Articuno. Look at their stats, here:
https://pokemongo.gamepress.gg/pokemon-list
(Filter by ice)

Articuno is the best ice attacker. Jynx is the only ice type with better attack stat, but she's too fragile.
Lapras is more of a defender.

Articuno, but depends on your number of rare candies and stardust I think...There is also little need for ice at the moment...

Mamoswine (gen 4, my guess is next year with gen 3 dropping this year) will make both of them obsolete assuming he gets a good moveset so you may want to pause vs. something like a Tyranitar which will be the top Dark attacker for a long long time.

I would go with articuno, since it has higher attack stats and about the same bulk compared to lapras. It's true that lapras has more stamina compared to articuno, thus it will accumulate energy faster during battle. But energy generation is on both side in a battle, this means your opponent will also generate more energy when fighting lapras. So you can fire more charge moves with lapras, but the same also applies to your opponent. It somehow accelerates the battle indirectly. But other than this, I would say articuno is a better ice fighter for the most matchups (discounting weakness for rock type moves)

i wouldn't spend rare candies on Lapras. Would rather use Shellder candies on a Cloyster!

Lapras. I wouldn't use super candies (is that how we say in English for the multicolar candies that we get against raid bosses ?) on Articuno as Mamoswine would be a much better ice attacker, as ironman_gaming said.
There is no instancy to get optimized ice attackers right now.

Wanted to add that having a maxed Lapras in the 1st gym meta (not maxed anymore), the rare candies are generally worth a lot more on probably another legendary than Articuno.

This is because among all the super hitting psychic types currently (minus Lugia who has 78 Attack less than Zam), they are all pretty glassy so Mewtwo is probably the 1st candidate to use rare candies on IMO.

Articuno probably doesn't bring enough to the table for me, but if I got 90%+ of all birds, I'd take them all to Level 30 too. I have now converted 41 candies to Lugia since I actually have a 90%+ one (actually 2).

I've powered up many pokemon that I later regret simply because I got caught up in what others said. So you should do whatever you feel like as that's the best policy I reckon.