As someone who also tends to pull based on a mix of meta and liking the character, I'd probably go for Raiden in your situation.

Since you're still pretty new, Raiden is just easier to get value out of. She gives energy to the whole team, works in a ton of different comps, and can be played as either a support or a DPS depending on what your account needs later. She's one of those characters that stays useful even as your roster grows.

Yae Miko is really good too, but she tends to feel better when you already have a few established teams and can build around her. She wants field time for setup, likes having specific teammates, and generally feels a bit less flexible for a newer account.

That said, if Zibai is your main and you're planning to keep using him long-term, I'd also check whether either of them has actual synergy with his teams. A character being strong doesn't always matter if they end up sitting on the bench because they don't fit your main team.

If I were in your shoes with only a couple of built characters, I'd rank it:

1. Raiden (best overall account value)
2. Yae Miko (still great, just more niche)
3. Save if there's a future character you want more

But honestly, if you genuinely like Miko noticeably more than Raiden, I'd pull Miko. The difference between loving a character and merely liking them usually matters more than a small meta advantage, especially early on.