Here's my take.
CMH/HID/HPS lights are viable choices-low entry cost, reasonably efficient. Primary downside is the amount of heat introduced into the grow environment. If you have the ability to get rid of that heat then they are hard to beat.
Now if you're like me, heat is a major hurdle for me to regulate. That is where led's can be a benefit. Using state of the art components like cob's or discrete diodes produce more PAR for each watt of electricity used. This means that less energy is wasted as heat. PPF(the amount of usable photons emit) gives you an accurate output of the light source and PPFD gives you an idea of how ell the lamp distributes that light over an area. Seems that the basic conversion from HID to LED is about 2/3-an efficient led lamp can produce the same amount of light of an HID. 650W of led should generate the same amount of light as a 1kW HID