To power up Raikou or not

Congrats.

I think it is a matter of what you want vs what you need. It seems to me that both raikou and mewtwo are not what you need for gyms or raids. So ask yourself which action will make you more happy and ask do it.

Try to get as many raikou candies as possible in the time remaining. I use pinap on low iv raikou with good success. But I would save the RC for mewtwo while gathering as many raikou candies as possible.

I would save for Mewtwo(timeframe unknown), while 100% is 100%, it'll still be there afterwards. You still have a chance (two weeks) to earn more Raikou candy and rare candy.

Compared to many posting here, I don't do that many raids. I've won 65 in total, and that includes the lower tier ones I did when they first started. Not counting the 10-15 rare candies I stupidly wasted in an attempt to level up a blissey in the first week, I still have accumulated enough to fully level up at least two legendaries if I so desired (and probably several more if I was only going up to level 30)

It depends on how much rare candy you have. If you have enough to power up the raikou and a future mewtwo right now, why not? It's not like there's going to be a better electric type coming any time soon.

If your Jolteon and Zapdos are serving you well enough for now, and you are hesitant to spend the rare candies, then wait until you have more rare candies.

If the Raikou does not already have Thunder Shock/Wild Charge, then at the very least wait until you have used TMs to switch it to its best moveset before powering up. Without Thunder Shock/Wild Charge, it will not be a big enough difference from the strength of Zapdos with Charge Beam/Thunderbolt or Jolteon with Thunder Shock/Thunderbolt.

I too caught a 100% Raikou and am taking the approach mentioned by gluglumaster, but solely for the purpose of leveling up a trophy.

You know you want to.