The one that best fits your situation. Lots of variables like budget, grow space dimensions, environment etc.
30-40 watts per square foot is comon with LED. Roughly $350-$1500 for 500 watts. HID is much cheaper but less efficient and may need more canopy to light distance. Less efficient means more heat for a given amount of light, an important consideration. Low 80s temp is good with led, several degrees less with hid.
Check out the led and other lighting section of RIU.