I use a helmet mirror. That way I only have to buy one for each helmet instead one for each bike
Seriously, my rationale is that a helmet mirror:
- Works with any bike
- Can be smaller for the same field of view, because of the distance to your eye
- Gives a better overall view since it's easier to rotate your head a little than it is to move your position around a bar mirror
- Has less vibration
One drawback is that if I'm in a tuck, all I see is my shoulder. I could adjust the mirror more to the side and have to turn my head more, but instead I just pop my head is a bit.
I tried eyeglass mirrors since I figured they would also give a pretty wide field of view, but the problem is that they were too close to my head, so if I set it so I couldn't see my ear, I'd have to turn my head more.
Also, I wasn't too keen on having to remove the mirror from my glasses after every ride. Though every now and then as I'm walking along, I look up and off to the side trying to see behind me in the mirror that isn't there. Or maybe I was just giving someone the side-eye
The generic helmet mirrors I got, something that looks just like
this- there are many sellers- may be a bit finicky to install depending on the helmet. I like that the ball joint is easy to adjust while riding since it is easy to knock out of place too. The hook and loop attachment could be stronger, but that's more because the mounting area is flat and it's usually on a curved part of my helmet.
Interestingly, the convex surface of the mirror does a pretty good job correcting for my nearsightedness, so I can see behind me even if my glasses slip down my nose.