This is an example of roll for design, not gameplay, since they fill a very similar role (as others have already said).
It’s like the current debate of Parvati over Valkyrie or (probably, though slightly lesser since I think OG Saber is widely viewed as better) the soon to be debate over Mordred vs OG Saber, though there are other examples as well.
They have very similar stats and kits, right down to the same NP effects, but their goal is slightly different. Ibaraki trades a very small amount of damage (their NP damage is almost identical, but Raikou will average about 3k more) and some team support for more staying power with the Max HP and debuff cleanse, and then Raikou has some Buster support while Ibaraki has team NP gain support.
So roll for design or the other servants on the banner, yeah.