I think it's just a personal preference. I tried the helmet mirror, but hated it....some I know love it. I have a nice round mirror on the drops (left side) that works well and doesn't stick out much.....it's probably about 2 1/2 inches in diameter and is adjustable, but doesn't stick out like the one on my mtb.

I think a mirror, either way, is a must for road riding safety.....