I've seen arguments for both. I probably lean towards Cora, for the fact that she had demonstrated better adaptation. Remember I'm 2x02, Regina struggled to adjust to New SB and wanted the book as a stepping stone again. Cora on the other hand showed no struggle to adjust to the NEW sb, seeing as she just travelled from the EF.
What are your opinions?
I feel that Cora is more powerful, because I feel like she has had a lot more experince and didn't really struggle in learning the concept of magic. She kind of clicked with the topic. While in that one episode, Regina struggled to rip the heart out of the unicorn.