What's the best supplemental light to use in a greenhouse?

[juggernaut]

LEDS OR HID'S TO USE IN SPRING/FALL/WINTER OR EVEN ON CLOUDY DAYS IN THE SUMMERTIME?
HOW MANY LIGHTS SHOULD YOU ADD PER SQ FT.

 

[juggernaut]

It's either HID or LED. Trying to get the best bang for my buck. For a 24' x 36' gh. Trying to figure out how many lights I will need for fall/winter/spring.