i answered your question on my website here:

dissidentsound

Headlights - Handlebar vs Helmet

bar mounted option is more popular but it is not necessarily better. rather every bike has a handlebar but not every rider wears a helmet, so ...

back when i used to do night rides here in US, the Dynamos seemed to be a German thing Germans love their Mechanical engineering ! i am neither in favor of or against dynamos. i think it may be good as backup in case your battery dies. for example you could have a battery powered main light on the helmet and dynamo powered secondary light on the handlebar or something. as everything German though i recall dynamos being expensive. batteries are cheap because they are Chinese, but they are a pain in that if you miss a season they may deplete and become inoperative whereas a Dynamo doesn't need constant topping up to stay functional. if i was German and wanted to support German engineering i would probably find a way to integrate a dynamo into my setup but if i recall correctly German Dynamos were never for high power lights like the ones used by Mountain Bikers. so if you're addicted to power like me they may not be for you. also does the dynamo powered light turn off when you stop at an intersection ? i wouldn't want to go invisible at an intersection !

Almost forgot - only helmet light lets you see tree branches before they take your eye out of course you should wear eye protection regardless @Prince Aleph