if you were stranded on an island would you rather it be hot or cold?
the thinner ones (when the usual glasses are very thick) are more expensive - but that is rather expensive so ask why - maybe a very good reason
A lot sorta depends on you, your style of riding and so on, so if you're riding a sports-bike and tend to let's say bend the rules a little than of course to want as much protection as possible but for the average person on a standard or cruiser just normal protection will usually be sufficient. The 'heavy armor' will only help in a go-down which is what you are supposed to avoid anyways if you ride to the conditions that the road dictates and will only provide minimal help if you were to collide with another vehicle, your helmet is your best defense in that case.
Duuuuuuuuh .hmmmm .uuhhh Its C! yahhh dats the one C!!