There shouldn't be much difference between helmet-mounted and glasses-mounted, but my own helmet, although quite comfortable, has always shifted around too much for a mirror.
Glasses-mount do require that the bow have a shape that will hold the mirror in place, ie, not round, as round lets it rotate around. My first one had kind of a ball joint with rubber, and it had to be adjusted frequently because it didn't stay put, and finally the rubber rotted out. What I and my family use now is the
"Beer-View" mirror which is made with a spoke or similar wire epoxied into a metal bottle cap with a mirror embedded down in it. It may take 20 minutes to get it adjusted right to fit your glasses and get the angle right; but once it is adjusted, you won't have to adjust it again unless you get new glasses, sit on it, or something like that.
Just to jump the gun and answer objections people tend to raise: It is nearly impossible to break, so if anyone is afraid of broken glass complicating an accident, the bottle-cap mirror would be the least likely to do that. Besides, the whole purpose of the mirror is to prevent accidents in the first place, something a helmet cannot do. (Saying the mirror adds danger is like not wanting to wear a seat belt because you think you might be trapped in a burning or sinking car, unable to get out.) I can tell you that I have, several times, when I was getting ready to go out for a ride, forgotten just how far the mirror sticks out there, and hit it on things like doors and cupboard doors, and it always either rotates away from my eyes, or it takes my glasses off and away from my face. The way my mirror is made, I don't see any way in the world that it can go toward my eyes.


