The reason i prefer helmet mirrors is their huge field of vision. Mine is only about 3/4" round, yet I get a complete view of what's behind me- not only the one lane but all the lanes, both shoulders, crossroad intersections behind me with cars turning onto the road behind me....everything almost up to both my sides! If i take that same mirror and put it on my bar end instead- i'd be lucky if I could see just the one lane right behind me to my left. It's sort of like the difference in car mirrors between what you see in your rear view mirror vs. your side mirror.

Yes, helmet mirrors take a week or two to get used to. But once you do, they are great. At first, it seems like they block your forward vision a bit on the left side where they hang near your temple...but your right eye quickly steps in to supply the missing visual info right there from its own angle, and soon the little mirror magically becomes "invisible" to you when you're looking ahead.