Alot of it come down to what you wanna keep. If you only want LPS, 24 leds would work fine. That would bring the retro kit cost down to $240. Dimming and non-dimming is a big cost difference aswell. The 36 leds would be alot more like 6 t5's or better. What kind of coral you want to keep? Do you want dimming? You looking for high light or similiar to your 4 bulb t5?