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.
> Modal windows can be used for views that don't constitute a "resource" or correspond to a domain entity: > [...] > Forms for creating/updating entities
Wow, in my opinion this a perfect example where a separate page IS still superior to a modal: how much better a "create issue" jira PAGE is vs the modal?