Just to give a different perspective, I use the helmet mounted mirror and feel naked without it.

I have also used the mirrors that clip onto your sunglasses, and they work just as well as far as visibility goes, but I had a problem with the little things that clip onto the sunglasses breaking off eventually, and they also scratched up my sunglasses. Maybe I was just too rough with them. I also wore my sunglasses for other things than cycling so had to keep taking the mirror on and off. If you could leave it on, it wouldn't be a problem.

The only down side to the helmet mounted mirror is at first, with the adhesive that came with it, it would come off sometimes. One time when it came off, a friend let me use some adhesive things she had, and it has never come off again. Unfortunately, I don't know what particular adhesive she had.