Which Electric Legend would you power up?

Zapdos 14/14/14 for elegance.

The possible return is right now with Kyogre raids. It's unclear whether there will be any other great raid matchups for electric pokemon in quite a while, as none of the legendaries left in either gen 3 or gen 4 fit that bill. And in gen 5, Zekrom has the stats to totally outclass both Zapdos and Raikou. Something to keep in mind.

That said, Raikou is great and is currently better than Zapdos. Not only does it have better DPS and tankiness, its single electric typing lets it potentially survive Blizzard from Kyogre which Zapdos can't do even at max level. Definitely choose a Raikou if you want to power up one of them.

Power up one of your raikou, useful to take down gym. Ts/wc,. Easy Dodge and strong DPs. Gen 5, perhaps is two years away. Raikou, personally feel like is the most useful legendary mon so far.

Difficult choice, but I think I would have gone for 13/15/15 Raikou. It is possible to reach the last attack point against Kyogre by power up to lev 40, so I would have chosen that.

If you are powering up for Kyogre...
Raikou with 15 atk IV hits a +25% breakpoint for Thunder Shock (4 damage to 5 damage) at level 38.5
14 atk IV at level 39.5
13 atk IV at level 40

Any lower atk IV will not hit this breakpoint with the current level cap.

Breakpoints for future matchups will be different, but generally since Thunder Shock is a low power move, higher atk IV will be better.

15 15 10 raikou

First - Congrats. You have some nice Raikous. Second - I would power-up the 13/15/15, assuming it had the highest CP at a given level. In general, in a situation like the one you have, I would power up the pokemon with the highest CP (at the same level).

Go for the ones with 15 attack. I like both of them. Raikou has better moves atm but zapdos has better attack.