Wondering is it better to have characters submitted through pm or through the OOC? Could you expand on why and why not for each?

OOC. With PMs you'd have to go out of your way to read each one. If they're in the thread, it's much easier to review them for acceptance + people can bounce off and see what other people are planning to make. This in turn limits the potential for overlap and might give your players an idea of what to write or if they wish to collab/write together with a character that would be of interest to theirs beforehand.

I would say OOC for reasons @Kuro mentioned, but I have had good luck having more sensitive stuff getting discussed via PM such as secret backstories and the like. I had a murder mystery RP that had public character sheets and a list of secret questions. Everyone had a lot of fun with the idea that not everyone knew everything about their character. But it is sort of a pain to manage if you don't wanna copy/paste the secret into a general folder.

As always, my answer is "It depends" but having characters in the OOC is good for a lot of reasons.