Are header navigation menu links, which occur on every page, considered to be internal links by Google, or would it be better for me to add same link to page content to get better page ranking?
Every link that points to the same domain as the one that it's on is considered an internal link.
Being linked from within the menu links on every page is likely to be considered more important than being linked from a single piece of content.
Just imagine you were Google trying to decide which page is more important:
- One that is linked to from every page in the header (so a lot of visitors will see it)
- Or one that is linked to from a single piece of content (so, in all likelihood, much less visitors will see it)