Personally, I would chose glass. The reason being is that it is more scratch resistant than plastic. I have had both and if you dont find some sort of case for a plastic mirror, it will scratch esialy from being bounced around inside a case. If you buy a good signal mirror, it should come with some sort of case. The other thing you can do is find some thin foam or packing material and use duct tape to make yourself a case for your mirror.
I have used the cheapo metal ones as well and they always seem to bend to easily and then I have problems aiming them.
Just my two cents,
Garrett
_________________________
On occasion of every accident that befalls you, remember to turn to yourself and inquire what power you have for turning it to use. - Epictetus