I agree that the units/ballasts may have similar costs, but in terms of 1-5 years of electricity bills, I'd be psyched with the difference between 400W and 24W! :-)
Actually, what I'd love to have are these new LED-sourced lighting that Amphibious has been talking about. I'd invest the extra money, considering the electricity bills would be drastically reduced (I've got 3 X 400W
MH for my new set up!!!!) AND the bulbs don't need to be replaced every 6-12 months. They're supposed to last for 5 years! I'm sure you get that extra initial investment back out pretty quickly.
