Both of them— the answer for me is both of them. 

I think there comes a point in planning loosely ahead and then letting the posts coming to you, especially in the GM seat as you never know how your players are going to react. You can try to predict them, but that plays on presumptions. I often like to play off ear, using my instincts guide me but as a writer I’m also fond of revisions and having some sort of start and end with the idea of the fun being how I get from point a to point b. Types of games are suited to variable approaches, but in most cases “have an idea, but never a rigid skeleton” seems to be best frequently.