Simply put, I think you are best of with custom save/cancel buttons. Not much more to it. The default save/cancel buttons really only do a commit/rollback with a close form. If you wanted to connect any followup forms to this it would have to be in object events and that can get really messy.
Custom buttons on the other hand give you the opportunity to change data, close the current form and open a new form based on input from the first form.