well this is a bit of a spoiler and was going to be its own thread when I get results, but I am running a little test I will keep out of details on brands and things until i get results.
So here it goes, when I was setting up my 125 my biggest unanswered question was what to do about lighting. I already had a 4' 6 bulb T5HO fixture and it grew corals out like crazy, in fact I burned an ill placed toadstool in less than a day. I was compelled to keep in the game of T5s as it was what I was comfortable with and I had success growing SPS with them. Then I started looking a new fixture and realized the price a 6' vs 4' bulbs was pretty substantial considering id need 12 per year. Then I began looking at LEDs and quickly realized it would cost almost a grand to cover my 6' tank with the ones I wanted. I debated with myself for a while on what to do since as we all know I had already laid down a pretty penny on a 125 with all the bells and whistles and the dang things isn't even stocked yet! after some long and rigorous maths that took me about 30 seconds on my cell phone, It was obvious I would be saving money quickly. So I boiled up a plan that I am so far pretty happy with. I decided to use my existing 4' T5 fixture to one end of the tank and an LED unit to the other side(1 of 3 that would be needed to cover the whole span.) From all of my reading I still posed many of the same questions you are before i spend a boat load on lights that didn't work for. All I will say about the fixture is that it uses huge 5w leds(hence quality.) During this test I will be using two species of each coral group to test the lights. I have not set a time limit yet but I am saying a few months minimum per side. I will be monitoring several aspects of the coral in judgement of health. This is kind of tricking since some corals respond quickly while others not so. These include but are not yet limited to, full coloration, polyp extension, food consumption, and growth. Again this will be kind of hard to follow since my best eating coral(a pagoda) is out of the test due to growing at a hilariously slow rate. Also most of my softies to not eat when target fed if at all that I have noticed and some feed at night. While these tests are not scientific methods by any means they are some of our most trusted coral health indicators in the hobby. A final testing area will be at the top the rock work where the tank receives light form both sources. test subjects(loose list)Softies: A leather(probably green finger,) and a zoa colony. LPS: Favia Brain and acan. SPS: monti cap and monti digi.
A major not is that these corals are not competing with one another and that they are going to be monitored individually based on the lighting they are under. Furthermore all corals will be placed under the same conditions at the same times yet not placed in close proximity. Also all corals will be placed in location that are optimal for them.
Now I will try and give you a more direct answer as you may have to wait up to a year for the conclusion to my experiment. I personally wrote of halides right away, now I know I will get flamed for this but remember I am not saying they don't and I realize they are probably till leader in growing light demanding coral and keeping deep tanks. To me they are like american muscle cars. Big motor, too heavy, and no handling(sorry guys i love my bimmers.) The connection I am trying to make is that while halides put off great amounts of the right kind of light they have some pretty strong(to me) draw backs. First of all they produce crazy amounts of heat that has to be controlled or you risk cooking you fish this is more money and possibly a chiller added in. Also they draw huge amounts of power and while my electricity is super cheap I do not like the idea of being wasteful. Finally, and this is case specific, I just dont need that much light to grown my coral, my sps live close to the top and do just fine. Now T5s have the most problems and also the most benefits in my and many other minds. Cons: still produce heat, expensive bulbs, large power draw, need good fixture to maximize light direction. Pros: wide spectrum of bulbs available, best light distribution over the whole tank, currently the largest market of support, no additional tank cooling needed in most cases. On to the LEDs, while most of what I said about the other two are established fact LEDs still vary enough to be a case to case basis. Generally speaking, modern system will hold multiple different spectrum's of leds which eliminate(so is said) the problem of not effectively growing corals. Draw backs, for starters a cheap LED units is likely not going to work out for growing much more than algae. Also the initial purchase cost of LEDs is pretty high but you don't have to buy bulbs. Leds are also very directional in lighting
So to sum it up, the answer is still it depends... My advice is to weigh the pros and cons of all three options and apply them to your wants and needs. My personal experience in the early stages has been very impressive form the LED fixture. It shines down through my rock work, through the bottom glass (BB tank) and illuminates my sump very well, which is not actually what I wanted but oh well. The shimmer from led looks cool but does nothing beneficial. Also the colors it pulls out of corals is stunning my Helfrichi firefish looks stunning in the morning and evening when the dawn and dusk actinics are on. I am still questioning its ability to produce natural light but it may no even be a relevant point if the coral does not care.
here is an easy down to earth summation. you are buying a new car what do you want? The muscle car talked about earlier that has the highest out put(MH.) a BMW 5 series, fast, comfortable, and yet agile and in the modern world, not particularly excellent at everything but still good at everything(T5.) or do you want a new hybrid sports car, sporty, dripping with technology and always changing, expensive to by yet cheap to run(LED.) In the end they will all work in most situations. My personal take is that LEDs are going to be the way but for the next five years or so they are going to continue to rapidly change. When these advances begin to slow and a norm sets in, I'll switch to all LEDs. Until then I will probably keep using T5 as my main lighting.
you asked my my $.02 and you get a whole lot more lol. excuse any errors I wrote this while doing three different things and I an tired.