Does the sticky tape really hold well enough for helmet mounts?

If I just get one light for MTB, which should I get? Helmet or Handlebar?

TLDR: totally depends on your use case, I'll elaborate...

Shouldn't I put the most powerful light on my helmet, not the bars?

Not necessarily, allow me to explain...

What do I do if the mount doesn't fit my helmet?

Why don't you include a strap mount for your helmet lights?