Bren: As you can see there are ONLY rules of thumbs.
In fact there are two each per responding member. I'll let you do the math.
Noise, weight, weather, bugs, comfort, "On/Off" road are all considerations in picking
a helmet, or choosing between
which helmet to wear.
Experience will generally provide you with the decision as to WHICH
helmet to wear.
Inevitably you'll realize that even your experience doesn't provide 100% accuracy of picking the "right" one every time.
It'll be windy and you'll wish you didn't have a visor, or dusty and wish you had goggles, or cold in a headwind of rain and wish you had a the full, or dual and not goggles, or just cold and you're heated and then (potentially) everything fogs.
As you may have gleamed from the many different responses, it is certainly of benefit to have options each having their own benefit.
My two thumbs....