I've used a handle bar mirror for a couple of years first on a leisure bike and now on my road bike.

I had the ones that velcroed on and don't care at all for them, I can never get them on tight enough and they seem always to shift, also it gets in the way when I go down in the drops. I am constantly adjusting it due to road chatter.

Advantage: wide range of vision when adjusted correctly, easy to look at

Bought a helmet mirror on Monday.

I am like the guy that wears both a belt and suspenders. I still have my handlebar mirror on until I get used to the helmet mirror

I think I will like the helmet mirror better in the long run because I'll have it no matter which bike I am on (I don't have a mirror on my old mtn bike and the road one doesn't fit), it is easy to adjust and stays in place