I will too answer that the best is to have both lights.
It's more failsafe but also give you a broader light spread. Different answers point out that lightsources being close to eye level make things look flat and reflection from mist, fog and suspended particles are more an issue, but also, handlebar mounted lights can cause long shadows behind objects not so tall, making it appear like a false hole. Some answer mentions it makes potholes looking bottomless.A low placed light is also more easily blocked by other riders if you ride in a group. Having both lights gives you the advantage of both and one is likely to overcome the disadvantages of the other at the expense of a few extra grams and an extra battery.
The helmet light is extremely handy if yo need to make roadside repairs, because you don't need an extra hand just to aim the light. For mountain biking I prefer the helmet mounted, because it is more stable (less shaky) than the handlebar. I've even had complaints from fellow riders that the shaky light is distracting. For night city riding the handlebar light is almost enough, but I like having also the helmet one to make me more visible to other road users, particularly when crossing streets, as you naturally look to the sides, you temporarily shine the light into drivers and pedestrians, which will draw their attention. Proper bike lights shine only a portion of their light to the sides, so they are less visible.
To help with eye level reflection, I recommend installing the light as high as possible on the helmet, this helps noticeably.