Poseidon, from what I hear from the R&D world, we may move right past LED and go right to plasma. Stay tuned!
Max2000: Both T5 and halide work. In your original post, there were some watt usage specs listed. I would like to see that comparison again, but including data for electronic MH ballasts. I think you'll find the data to be a lot closer.
I also wonder about the cost... I paid nowhere near $1000 for my dual halides with actinics retro, and I didn't get the cheapest option out there.
What do you plan on keeping in the tank? That drives the number of bulbs and the required intensity.
Having said all that, I think it's hard to consider anything but T5 for a tank of 4 foot width or less. T5 is cooler and more cost effective for those applications.
T5 systems do put out heat! You do need fans to keep the air moving, else the output will diminish, and bulb life will shorten as well. I have used setups that came without fans, and it ran hotter than my halides.
Hopefully your tank is not the 72 inch length. With T5, most people use the 3 or 4 foot bulbs over tanks; for the common reef sizes of 36" or 48" wide it works fine. Even a 60" tank can get by with 4 foot bulbs.
A 72" tank won't be able to use that size.... and even though there are 5 foot T5 bulbs available, their output isn't as strong.
That means you'll need to use a system that has 3 foot bulbs, with a bank of 4 to 7 per side (based upon your needs). That is a LOT of bulbs, and can get expensive at changing time. It's also a bit complex and a space hog as well.... and gets harder to cool.
Remember that reflectors are key to T5 output. Get a system with individual reflectors for each bulb. Essential!
Also note that there's a lot of CHEAP T5 systems out there... and that means cheap in terms of quality. Try to stick with name brand products with good ballasts....else you won't get the performance or reliability that you'd likely expect.
