See also: What is the relationship between a Microsoft Word document and its template? and What happens when I attach a new template to my document? or How do I copy content and settings from a template to a document? by Shauna Kelly MVP.

Following your first mentioned procedure of a new document gives the new document the content and structure from the template (page size/orientation/margins/background/styles). Attaching a template can give you the styles and access to macros/QAT mods/building blocks but not to the underlying structure or content.