Who is the better servant to save for

Merlin of course

As you said, Merlin is broken. If you don't like any other waifus, go for Merlin. Raikou is a really good farming servant, but you can make on pair farming teams even without her. Support servants can even carry you through farming. Good luck!

I think you simply answered your own question with 'Merlin the most broken caster's there you go.
Raikou is a good servant, but remember she's a berserker she might have better NP gain than a normal one, but a simple crit on your enemy of if the enemies target her only in a turn all that charged NP is useless.

While Merlin is indeed broken, he doesn't simply make "any Servant strong". While he's leaning on the side of a supportive Caster, it's best to spam his NP as much as possible, where, coupled with his skills, he gets his memes.

I think you asked this cause you wanna hear Raikou as an answer. While clearly Merlin is the better unit, do note that you will almost surely have access to Merlin 10 10 10 as support as so many are gonna roll for him. That is, Merlin is a better unit no matter what. But if you want Raikou, go for her. Waifus>>>> Power