How can one differentiate the usage of -tte and sou desu when telling what someone is saying or has said? I consider them to be interchangeable. At the same time I do not see how they are different from "to iu (says)" "to hanasu(speaks)". Is there something I need to know? I know the usage of all of them, just wish to confirm that they are interchangeable or not, and any restrictions I need to be on the look out for.
