LED's have come a long ways.
While I've only been in the hobby for going on 5 years, I've done lots and lots and lots of research.
I've never personally ran Metal Halides myself, so I can't speak from personal experience with them. However I've had many close friends here and locally run metal halides.
Metal Halides are incredibly powerful lights. They have been proven with success for over a decade with incredible results for all sorts of corals. They provide great spread with just a single fixture. Corals can grow comfortably knowing they have plenty of light all over them.
Now while Metal Halides are powerful and proven success, they have some obvious downfalls. They run quite hot. There are clear ways to obviously reduce the heat transferred from the light to the tank, however those methods aren't always the most cost friendly. Aside from that, the units are generally big and bulky. Granted you have a few fixtures that are oh-so sexy, like the Giesemann Spectra and Infiniti with built in T5's. Though others are usually a bit bulky with a big reflector. They do the job, well, however moving into the newer age, people seem to be wanting more sleek and contemporary-ish looks.
Acclimation for Metal Halides usually revolves in turning on/off for shorter periods, gradually increasing the length of the time you run the fixture or lowering the fixture over time.
LED's are still obviously new and upcoming, though they're continuously proving themselves that they can produce similar and sometimes even better results in coral growth and color. The LED's run a lot cooler and much more efficient. Living in a house in Las Vegas for example, won't leave you with heat issues during the peak of summer. Aside from that, most LED's are completely controllable. Some you can control two colors, some you can control the color and intensity with the spectrum already programmed in, and some you can completely control each individual color the LED fixture offers. Not only that, but you can ramp them up/down. While some come with controllers, others don't. But this will allow you to create your settings everyday. Some allow you to have lightening storms and even introduce certain %'s of cloud coverage to make it that much more real. Cool stuff, right?!
Kinda.... The issue we get into here with all the cool controls is... Well.. The controls. It's all fun and games for you, however your coral will suffer from the intensities and spectrum changing often. It seems people who jump into LED's wanna try the new profiles, or what Joe Blow is running over his tank because damn, he's got some awesome success. That's cool that they want to try it, but most give it 1-2 months and think hell, this isn't working. Lets try something else. Well wrong! Unless your corals are burning and dying or reaching for more light, don't adjust it. Set it and forget it. Corals will adjust to the spectrum and intensity. Eventually they will get comfortable and take off in growth, all set aside from parameters.
The other issue with LED's is coverage. Now while you only needed 1 metal halide over a tank for SPS dominant (talking larger colonies), you may need 2-3 LED fixtures. SPS colonies especially will usually suffer under LED's because a part of them underneath is not receiving any light due to shadowing. This usually means more $$$ for another fixture to help increase the areas a coral will receive light.
Acclimation for LED's has two-ish options. 1) You manually increase the % of the intensity every 1-2 weeks by 3-5%. 2) Some come with the option of allowing you to use your settings the way they are, but automatically increase intensity over the course of 3-4 weeks or however long YOU choose.
I personally have ran only T5's and LED's. I've had amazing success with both. The key for LED's is, since they're not plug and play, find a profile your corals seem to enjoy or most people have success with and STICK TO IT. Don't change it or anything like that.
In conclusion, both will do exactly what you want. Both will grow and color corals incredibly well, given the right parameters and etc.. You'll have to personally write out what YOU are looking for a in a light and what YOUR budget is. Do you want something thats plug-n-play but gives off lots of heat and don't mind replacing bulbs? Or do you want something you can have a sunrise/sunset with to enjoy your tank longer, play with settings, and can have much cooler running fixture?
If it were me, like I did just recently, I'd go with LED's. To save money, you can buy well taken care of fixtures for $1-300 less than you would brand new that include mounts and all that good stuff. Though you will hear many mixed reviews.
Either way, YOU WON'T BE WRONG!
Cheers and happy new year!
