Some preaching methodologies suggest that the main idea is what crosses from textual study to sermon preparation. Others suggest that the outline of the text crosses over to form the outline of the sermon. Which is right?

Both, but with qualifications.

The idea is in charge of the message, the outline is not. Remember that the main idea of the passage was what the author was seeking to communicate to the recipients, and he chose to do so making choices about genre, structure, details, etc. Everything after the idea is a matter of authorial strategy. As we prepare to preach, our goal is to firstly grasp the main idea of the text, process that idea so that it takes into account the needs and situation of the listeners, and then consider how to form a sermon that will effectively deliver that main idea.

The outline of the text is not boss, but it does matter. In my approach, I teach a narrowing focus in the textual study that culminates in the defining of the main idea of the passage. That idea is then influenced (in certain respects) by an overt awareness of the listeners which determines the purpose of the message, and then the message idea is then in charge of the subsequent decisions relating to strategy (including the message structure, the illustrative details, intro, conclusion, etc.)

Having said all that, when it comes to the structure or outline of the message, where do I begin? With a contemporised outline derived from the passage. In effect the work done on the idea is also done on the outline.  So why don’t I overtly state that in the 8-stage process?

The outline of the passage is a starting place, but it does not always have to be obeyed. My default approach is to follow the strategy the author used by following the order and structure of the passage in my message outline.  However, I don’t feel restricted by this approach.  Sometimes the contemporary listeners are in a different place to the original recipients.  Sometimes they need differing strategies to drive the main idea home.  Perhaps extra info is needed, or a different starting place, or perhaps a different ordering of the content of the passage.

The passage outline is the place to start when it comes to the message outline, but it is not a requirement.  (However, I do feel constrained by the main idea of the text as I work at the level of main idea – hence my approach that emphasises the progress from passage idea to message idea).