Followups vs Rules

Hi i have a question where is the diffrence between followup actions and a rules? It seems like there are pretty similar so what is better to use?

I have a case where my action in one case should a form be triggered and i am considering now just to setslot in this action like pending_action and then model it in rule with slot_was_set then trigger the form or i directly set the form/action with a followup