I got a Rainbow Blight waay early in my game. She has been my go-to can opener for every difficult fight since that moment. She's lv 58 diamond, and I just don't understand why she didn't earn an SS rating on attack. Her buff/debuff roulette makes her competitive in almost every scenario and fight modifier.
I got Bloodbath much later, but playing her is a treat. She hits hard, heals herself, pushes bleed onto her opponents, and generally has a lot of style. Certain tough battles become laughably easy with Bloodbath. She has fights which do not suit her, so one must pay attention to the modifiers.
I love them both. I would not deny anyone a chance to sleepwalk through Caturday, but I would favor Rainbow Blight for her sheer utility.
But save the relics you buy; I suspect holding onto them until 3.0 drops is potentially worth the wait.