Beside different mounts, the mirror shape, size and convexity are also to be considered. Strongly convex ones might be good in city traffic as they give a wide field of view, but for seeing distant (but fast) cars on country roads a flat mirror is better.
Also consider that some mirrors may make the handlebar wider, which can be a problem when filtering in traffic or on narrow cycle paths or shared footpaths, but isn't an issue on country roads.
In practical terms, as everybody's bike, body, riding style and road environment is different, it can be very difficult to find a mirror or mount that works well for you. I tried literally dozens. I recommend, before buying a good expensive one, buy a cheap one in a 1-dollar/1-pound/1-euro shop and tape it to the handle bar in different positions to get a feel how much you can see and where the best mount point would be for you.
Besides bicycle mirrors, it's also worth looking at motorbike, e-bike, mobility scooter and wheelchair mirrors. I'm now using a motorbike bar-end mirror (aluminium frame with real glass mirror), which has the advantage that it's more solid than any bicycle mirror that I could find and doesn't wobble or fall off at potholes (of which there are plenty here), and also has better optical qualities.
