This question cannot be answered because the voltage is of little importance unless you know the configuration of the light, modern LED emitters use less than 6 volts. One can use many emitters in parallel or in series (in series requires higher voltage) many super high power emitters have multiple dies on a single substrate connected in series so their lowest voltage is more than 3V because the series/parallel arrangement is within the emitter itself and cannot be changed. So the best system is the one that is closest to te requirements for the emitter you will be using or the one you want to use in the future. You could say the best system is the one that can deliver the most Watts (V x A), but running more A causes more heat and requires more expensive design and components, so if you can match a higher voltage source to an emitter(s) that can deliver more Lumens, then you might be better off with that one. So really it has nothing to do with 6V or 12V, but which one has the better design over all, and the best design is the one that fits your requirements the best.