I ride 24 hour races (teams and pairs) and use Ay Up lights, if you plan on riding anything technical then you really need both a helmet and bar light as the light from two different directions gives much better depth perception. Also on twisty single track, a bar mount won't necessarily be pointing at the track ahead if you're cornering so you can use your helmet light to scan ahead.

With Ay Up lights you can adjust the angle super easy so I angle my bar light down to a few meters in front of my wheel and the helmet light angled so it lights up a bit further away. The helmet light follows all your head movements so lights up what you instinctively look at but the bar light keeps a bit of light in front of your wheel for those pesky rocks that come out of nowhere at night.

With AyUps you can put the same light on either a bar or helmet mount so you can always switch the light over if you only buy one.

For street commuting or road riding I just use the bar light only, more than enough light.