Live conversation is an exchange. You speak, the model responds, and you can ask a follow-up question. Use it to examine an idea, explain a problem or rehearse an argument. State what you want from the exchange before you begin.
Dictation turns speech into written text. You can dictate into a document or compose a message for a model. You then decide when to review, edit or submit the text. This separates getting the words down from asking for a response.
Check which interaction your chosen tool provides. A microphone button can mean either one. If you need a transcript, check that you can read and retain it before starting.