I have used a helmet mounted Third Eye mirror, and a Take A Look eyeglass mounted mirror, and yes, I wear bifocals. (I don't need them, I just wear them to look smarter ).

I much prefer the Take A Look.

A tip on the Third Eye- we keep our helmets in the garage with the bikes. In the summer, the heat would soften the glue, to the point where the mount would slide/move.
A small drill and zip tie fixed it.

ALso, the ball/socket on the Third Eye eventually (several years) got to the point where it would not tighten any more, and would slowly allow the mirror to drop down. I would have to dis-assemble it, wrap the socket sleeve in a piece of plastic wrap and re-assemble every so often.

The Take A Look also has a helmet mount of some sort available- have not used it though.