All decisions should be made with an eye on the future. You won't get Karin unless you roll on her, either with Jewels or money, or pick her up with a Miracle ticket. Rolling on her means sacrificing rolls on something else, like Kirin or Mammon. Although, sadly for Thunder, their +100% eidolon doesn't coincide with a decent SSR kamihime, or even a Thunder SSR at all. Kind of got screwed there.