I did both but usually preferred to plan 1-2 topics for each session. It still often diverged all over the map as I have a tendency to ramble when I don't need to control it. I think both approaches can have advantages, e.g. with planning and focusing it is easier to follow through and tackle goals, and with spontaneity it's more likely for unexpected, normally suppressed and controlled things to come up. I think it is best to feel free in therapy so if leaving it to moment is more natural for you, do that, why not. Or you can alternate the strategies for different sessions and see, from experience, which is more helpful. I imagine that's what most people do in therapy by default unless it is some kind of very structured, focused modality, especially short-term or the idea is to just express any feeling.