Artificial growing lights are simply trying to replicate the best parts of natural light. The full spectrum of light provided by the sun is needed for best growth so natural light is better if you are just comparing which emits the best wavelengths of light. Artificial lights can make plants grow faster though because you can control exactly the duration and intensity of the light. You can in effect create 24 hours a day of daylight maximizing growth. Natural light is less efficient since the angle of the sun, number of daylight hours, clouds, etc. can impact the plant's growth cycle.