Which Raikou to power up?

15A one. No point other one since you can’t leave them in gyms.

The first one has slightly more survivability, but that only affects it taking charge moves unless it's hitting defensive breakpoints (which are harder to gauge)

Dont power up any attacker that doesnt have 15 A . IMO

If you plan on taking it to 38.5 or 39, and have the dust handy. As you said there isn't any meaningful difference on 14-15.

If you stopping at 30, or too low a level to hit the top for a while, then you have to decide if its worth doing without the 1 attack short term

I don't think there's a wrong answer here. You won't notice any difference between the 2 I think. That being said, with them being so close, I'd go with the 15 attack in case it hits a breakpoint more easily against a future boss.

1st one.

I would power up the 98 because it has higher cp, assuming Raikou is no longer in your rotation. I just can't stand powering up a lower potential cp mon of the same species.

The 98 one. That 1 point lacking in attack, you solve giving it one more power up to reach whatever breack point.
(The chance of getting a 98% is around 1,4%, be glad dude)

I powered up a 98% 14a/15d/15s, but my best with 15a was only 89% so not as hard a decision. Your two are incredibly close and the difference probably doesn't matter too much either way unless you are trying to power up only to meet a specific breakpoint against a specific raid boss and don't want to go any higher. Their attack and defense stat breakpoints are within one power up of each other, so unless a breakpoint falls at level 39-40 you can use one more power up to reach the same functionality.