I used a helmet mounted mirror years ago when I could be seen riding my red Trek 1500 road bike. My old helmet mirror and that road bike are long gone. Now I only have a tri-bike. Instead of buying a new helmet mirror I decided to try something different. I purchased a Zefal Spy mirror. The Zefal Spy has a slightly convex mirror that is designed to mount on the handlebars. The convex mirror provides a wider field of view than that of a flat mirror. It also makes objects appear further away than they really are.
|Zefal Spy mirror mounted on Vision Carbon Wing base-bar|
By contrast, from what I remember, a helmet mounted mirror allows for easier adjustment and the ability to focus on objects behind you. Simply turn your head until the object comes into view. A helmet mirror probably has a wider field of view than the Zefal Spy as well. However, like the base-bar mounted mirror, vision would be impaired when using the helmet mounted mirror due to the two riding positions.
So, neither mirror provides the perfect solution. Maybe using both mirrors would do the trick. I'm disappointed that the Zefal Spy mirror didn't work well for me. I think I'll give the Zefal Spy to my wife and abscond with the helmet mirror that she won't wear.