W3Schools.com and I\'m pretty sure I remember seeing W3C.org state that should be used for Toolbar menus and listing form control commands.
is a section (like or ) so it will appear in your HTML document outline. Due to this, I would use 99% of the time. To me, is a mix between and , with the added constriction that should be used specifically for navigation, whereas can be for anything that might be called a menu (including a navigation bar). So most navigation bars, despite the fact that they are navigation bars, do not fit the strict requirements of , and is more appropriate.
I have never in my web development career found a situation where I wanted my navigation bar to be part of the document outline.