I don't know what all these guys here are talking about. Serious growers grow outside first of all, HPS is good for weight, not great for quality. Metal Halide is better for Quality and worse for weight (although I don't entirely accept the weight argument with that). Both use a shit load of electricity and produce a troublesome amount of heat.
I own both HPS and MH. I love them both. But I want better quality from my bud, and that means UV radiation. HPS
SUCKS at emitting UV, especially thorugh the glass protective panel. UV = Trichromes, trichromes = THC. Metal Halides, with thier smaller wavelengths have a considerable UV output, but lack the longer red wavelengths that HPS offers. Those longer wavelengths make for better bud production and allegedly "denser" buds.
LED lights are something completely different. Each individual diode has a specific wavelength, some blue, some red, (if you get a serious panel) some UV, some IR. The lights sip electricity, most actually consume less than what they are advertised at. The heat they produce is negligable at best, the max surface temp on my heatsink as well as the surface of the LEDs is 95 degrees. You can imagine that (unless your growing in a small air tight space) heat is not an issue.
Now I come to why I use both. I bought a nice LED panel, 500W (280W actual draw). The thing is so bright. I started my grow in my basement, and things were going real slow. Im about 2 days I figured it out. My ambient temp in my basement is 60 degrees. In my grow room with my HID's on it gets to 82 degrees, which is perfect. However, with my super cool LED's the temps never really tilted past 62 degrees. Obviously, with the temp gauging the plants metabolism, things were growing really slow. Seeing as how electric heaters pull about 1500W, I figured I would just keep my HID on and toss my LED with it.
Best idea of my life IMHO. I get the Weight of the HPS and the quality of the LED. Link in the sig.
LED's work great, especially if you have a tight space to grow in. There are some reasonable prices out there too. I use blackstar LED's (cheapest quality lights I could find). Where you win out on LED is on g/watt.
In my eyes, LED lighting is REALLY overpriced.
REALLY. I spent 750$ on 740W (actual pull = 385W) of LED lighting. There are light out there that are 240 bucks for 35 watts.
Good read on LED lighting ->
https://www.rollitup.org/indoor-growing/240615-led-users-unite-228.html#post5311703