I ride a road bike almost exclusively and use a helmet-mounted mirror. I just adjust it to be able to see from my normal position on the hoods. If I'm in the drops and looking ahead I can still usually see what's behind me.

With a bike-mounted mirror, you're only going to see what you can see in the mirror's field of view. With the mirror attached to your head in some way, you can scan around and see a much wider field of view behind you.