I have tried several kinds of mirrors. My two favourites are the MirrCycle that mounts on the end of flat bars, and the Take-a-look mirror, which you can use on glasses or helmet (I use on sunglasses). I had a third eye helmet mount which I could never get into a useful position, and flimsier bar end mirrors. The MirrCycle is quite sturdy and stays in place. You may need to very occasionally tighten it with allen keys if its been bent around a lot.

MirrCycle is always on my flat-bar commuter bike, so if I don't wear my helmet or sunglasses I still have a mirror. It does make the bars a little wider which can be annoying for tight traffic or spaces between houses to roll my bike into friends backyards.

I use the Take-a-look with my drop bar bike and folding bike. I like that I have a more complete view of the road because I can turn my head to sweep it. It sometimes requires adjusting because it will have gotten a bit bent just sitting somewhere.

For $15 each (more or less) you can't lose, pick one and if you don't like it, try the other.