Which one to power up?

wait for a 15att

I would be happy with either of those. The choice won't make a noticeable difference in its performance. If you invest in the 15 def, there may be some match-ups that prevent the attacker (depending on it's attack) get a break point (either in its fast or quick move)

If you invest in he 15 hp it will have one extra hit point.

The decision will probably give unnoticeable advantage in certain match-ups and the other will provide said advantage in others.

It don' matter

If 'till the end of the legendary raids you don't catch a better one, I'd choose the first option. Stamina is Zapdos worst status, so considering that they have the same ATK, a 15 HP is the best choice in my opinion.

Zapdos cannot last long as an attacker as it cannot handle damage if you don't dodge.

I'm 3/3 today 1891, 1855, 1884...I already put a little into an 1888---2500stardustx2 obviously isn't a big deal but gonna take a few of these above 2500cp and then go big with the best one