I would say I do both, as I usually plan out an adventure but will throw it to the wind, as the players go in different directions. And usually my planned adventures are very light on info, so I have to adlib on the spot often anyways.
Yup - I call it Plot Fishing. When we're either at a lull or just starting, I'll dangle a half a dozen plot hooks in front of the group. WHen they bite on one I yank.