both have their pros and cons. plenty of evidence on RIU of how good led can do if you search. In a space that is only 4ft high, I will say led is going to yield more than hps simply due to it being easier to control environment and temps. in a space that is only 18x36" its also going to be easier to get more even coverage w led due to the height restrictions. cost in diy led vs hps is minimal in a space this small. if you have the space for 5400w of hid, its a totally different discussion. lighting all that w led would be more efficient and cheaper on the electric bill, (assuming that each 600 covers a 4x4 area, you could replace each 600w w 480w of led) but a lot more expensive up front.