Mirrors come in several styles I will discuss each one.

1. Helmet mirror: These are small and light. Due to vision problems I have never been able to use these. Hence I am unable to give an objective opinion. However they seem ideal for touring, just try them out before you buy.

2. Frame mounted or bar mounted ( with straps ): I've had several frame mounted & bar mounted ( with straps ) mirrors. The most interesting mounted on the frame between my legs, I had to look straight down to use it. All of these mounts were unstable and allowed the mirror to moved, hence they never worked properly. Stay away from this stuff.

3. Handlebar mounted ( attached with a allen bolt ): I used a Mirrycle about 10 years ago and fell in love. These are perfectly stable and can easily adjust to any angle. It can even be used as a bar end on the left side. The rest of the market is catching up but Mirrycle is still my favorite for touring. 

Overall helmet mirrors seem ideal if you can use them, otherwise go for a Mirrycle.