Hi there
According to Google, it really doesn't matter, but I like the idea of everything being clean and equity being passed, so I would wait until the https version is live and your migration is complete. Reason being, I want that equity to be transferred fully to where it's supposed to be, especially in the destination is leveraging a ranking factor like https.
I would follow Google's migration resource and make sure you are doing this with best practices in mind. I would also look at your backlink profile to make sure you're not transferring bad backlinks for Site A to Site B in your move.
Hope this helps! good luck!
P