I think this is a toss up. This morning I thought I found the ultimate excuse for helmet mounted, but I was wrong. With a helmet mounted light you can shine the light any place you need, including at cars backing out of driveways.
Also with helmet mounted you can use multiple bikes, without swapping the light back and forth. The one downside to helmet mounted is that I have a helmet cover I really like for cold or wet weather. Those two things are mutually exclusive. Therefore I think it’s helmet mounted in the warm months and bike mounted in the cold months.
I use a cygolite milion. It is bright enough to actuallly see the road, yet self contained so it is simple. The battery is rechargeable, and you plug the light into a standard mini USB to charge it. No extra charge brick. It comes with bike and helmet mounts. It is water proof to rain. I’d buy another.