I have several bikes and the handlebar mounted mirrors had to be mounted in different places (you can't use a bar end mount if you have bar end shifters). Also, my left arm would get in the way of seeing the mirror for some mounting positions. I tried a helmet mounted mirror several years ago and gave up. At that time I was still wearing bifocals and one has to view the mirror through the edge of the lens where there is a lot of distortion (too much to be any good in my case). Since then I have had cataract surgery and have 20/20 for distance. I still wear sunglasses (for eye protection) but they are uniform thickness and do not cause distortion.

There are several different brands of helmet mounted mirrors. I use a "Take-a-Look" mirror.

Some mirrors (both handlebar and helmet mounted) de-magnify; so if you spot a car in the mirror it's actually much closer than it appears. The Take-a_Look is flat mirror, so you get a good sense of the distance.

A couple of photos to show how it's attached: