1love1earth said:not sure which HPS bulb is the "best", there are so many different ones out there, dual arc, hortilux, soooo many, but a 600 watt digital HPS is going to be the "most efficient" proven method.
Certainly LED is efficient, but super expensive.
CFL is efficient as well, but if you want to grow solid weight easily, I'd say the 600 watt digital ballast HPS is going to be the best.
If you're looking at a smaller grow, a 250 watt CMH or such would work great.