Big fan of largish helmet mirrors. specifically the Cat Eye one with the short stem and the screw clamp. But I don't clamp it to the helmet. I make a bracket that screws to the helmet with two small (#4) screws. Bracket is made of sheet aluminum bent to fit the helmet and with a lip at the bottom so the clamp cannot slide off. I then fiberglass both sides. Clean up, drill for screws, done. Lasts until I can no longer find a helmet it will fit to.
With this approach, I can make brackets that put the mirror exactly where I want it. I love it. I've been doing this about twenty years. (Thank you Cat Eye for keeping that same mirror in stock. I may even reuse an old bracket if and when my current helmet goes out of style.)
For me, the real reason for mirrors is to know what's coming so I can make the best choices on how to handle what's in front of me. With a helmet mirror, that look is fast and I barely have to take my eyes off what's ahead. Also I can "wander" all over the bike and not change my mirror access and view. (Push back all the way on the seat to power climb, "ride the rivet" very far forward, stand, ride no hands ...) And (I'm sure this has never happened to anybody else here) I have crashed enough that I cannot turn my head fully to see behind. I have to use enough shoulder that it affects my balance and riding a straight line is hard. Racing days - I was taught to master that seamless look back. Last season of racing after my "big one" I could no longer look back over my right shoulder. I raced that season planning my ride so I didn't need to know who was there. (If possible, I made moves from the right edge of the road where I could monitor everybody.)