Yes, heat is a concern, was looking at a used metal halide / t-5 fixture for 250 bucks. I love led's but have heard from too many people that the technology is just quite not there yet. Maybe in a few years, but not now. Also looking at prices of led's, I am not sure I could get into anything that is remotely close to being able to handle keeping ops happy for under a grand.
Yes, you'd need to spend about 1.5k to 2k to completely light a 4 foot, deep SPS tank with high-quality LED fixtures, though you could spend considerably less if you are OK with the Chinese imports.
I struggled with the same choice when I set up a 50 gal rimless cube a few months back. I started the hobby with MH and VHO in the early 1990's, and still have a 20 gallon nano illuminated with high output PC lamps.
This is my take - an existing tank that is currently lit with fluorescents may have some difficulty switching to MH or LEDs only. Fluorescents have a very large illuminated surface area, so there are few shadows in such a tank. Metal halides are closer to being a directional point source, but still have a fairly distributed light output. Modular LED fixtures like the Ecotech Radion, Aqua Illumination Vega/Hydra/Sols and the Maxspect Mazerras are highly directional, so they will cast shadows that may result in shaded portions of existing corals dying back.
That is why I run 2 Vegas over a 2 foot by 2 foot tank - there are far less shadows than using one Vega alone. However, I'm still thinking of adding a couple of "Build My LED" striplights to the sides to better distribute the light.
Pacific Sun and ATI LED Powermodule lights are designed to get around this problem by incorporating T5 fluorescents with high-output LEDs, and if I had it to do over again I probably would've gone this route or used an Orphek Atlantik fixture.
All that said, here's the cost calculation:
LEDs need no lamp replacement - they will last 5-7 years at which point the technology will have improved to the point where you probably wouldn't want to repair the fixture. And they are about twice as efficient as T5 flourescents and about 3-4 times as efficient as metal halides in terms of watts used per lumen of light output. T5 fluorescents have perhaps the highest on-going lamp replacement costs per lumen of light output, but are usually still cool enough that a chiller isn't required. Metal halide lamp replacements are a good bit cheaper per lumen/area covered in the tank. But they are electricity hogs and send significant amounts of infrared into the tank, so you may be forced to run a chiller in the hotter months or keep your house/room AC in the 72 - 74 deg F range to keep the tank cool.
So my take on it is that even high-end LED fixtures are a good bit better deal than metal halides and fluorescents in monetary terms over a 2 or 3 year time period, even when the initial purchase price of roughly 2X over a high-quality T5 fixture is considered.
But my own experience suggests that LEDs have to be set up very carefully and the coral placement needs to be precise to make sure something doesn't get excessively shadowed. Metal halide lighting is easier, and fluorescents are pretty much idiot-proof. But it's at a cost of a great deal more electricity usage.
By the way - metal halide fixtures and ballasts are definitely not created equal. There's a very large difference between a high-quality, high-frequency digital ballast like a Lumatek, an electronic ballast like a Hamilton, and an old-style magnetic ballast in terms of electricity usage, lamp life and heat output. I would not personally choose to pay $250 for a used MH/T5 fixture if it's more than a couple of years old.
