If users are expected to spend a long time and use many tools to compose something, then I agree a full page form is best. For something like adding to a todo l

If users are expected to spend a long time and use many tools to compose something, then I agree a full page form is best. For something like adding to a todo list, or where users are expected to add many items one after the other, a modal is better. You can also take a hybrid approach: many calendar apps have a small popup for adding an event that the user can expand into a whole (nonmodal) window for editing all the details.