I do both, but pretty much on a case-by-case basis.

I may have the RP planned out weeks or even months before, but what if my partner has other ideas or new inspirations I can use/add into the mix?
I may not have the RP planned in detail - just the basic skeleton to get started - but what if my partner decides to take the backseat?

It's hard for me to stick to either one of the habit for all the RPs I do, so depending on the premise of the RP idea and the kind of partners I have for the respective RP, I'll need to be flexible so that I can work out something that both me and my partners are comfortable with and able to keep it going for a longer term.