I had 3 40gl frag tanks a few years back that were running Rapid Led Onyx fixtures - 2 on each tank. I was growing mainly monties and sps and was absolutely amazed at the growth and coloration. I have always ran high alk/cal/mg/flow tanks and had a pretty good handle on the tank nutrients - if under MHs I would have expected the same thing. Nitrates stated getting a little high in the tank and I foolishly installed a Bio Pellet reactor without understanding just how good a job they could do with nutrient control. Long story short - after a couple of months the coral really started to suffer so I sold them all and blamed the LEDs ;). The LEDs were fine - I had a lot of coverage and ran them cranked up. The corals grew like weeds. I killed the ctanks because I was careless with testing and didn't stop to think it out. Not the LEDs fault. No chiller required.
That being said, currently I use both MH and LEDs on different tanks. I think they are both great. My MH tank is primarily SPS but I have various LPS in lower light areas including a bunch of rainbow chalice - all of this is under Radiums with par at the top of the tank running in the 600s and about 300 at the bottom of the tank. The MH are supplemented by 4 T5s and 3 Reefbrite HOs. I run the MH 6 hours a day and the T5s & LEDs 11. The chiller comes on about 6 times a day for a few min so the noise and cost are kept to a minimum. With respect to bulb replacement and regardless of all the talk about replacement every 6 months, I easily get a year out of my bulbs and only replace them when I show that the par has dropped off 20% or so. I have gone as long as 2 years with Radium's and still had 70% of the initial par when new. Once most MH bulbs have been ran for 6 months or so the par values will drop off by around 15%. By the end of the first year most will still have 70% or so of their initial par and over the next year will see par values decline very little. In my case that means that 600 par @ 6" would be 420 par at 6". Its a respectable number none the less and of course I could simply lower the light a little to pick some of the par back up. So I feel its safe to say that you can forget the bulb replacement argument.
http://www.reefkeeping.com/issues/2007-04/ac/index.php
At my office I run a RedSea Reefer that has 2 Hydra 52 HDs. Its a new tank - about 6 months old. The tank is primarily an LPS / Zoa tank though I do have a few SPS at the top of the tank. Starting at 7AM I run the Hydras at 30% and ramp up to 70% by 11AM and keep it there till 4PM and then ramp back down to 30% by 8PM. We are ALWAYS work at late so I want to see the tank lights on when I'm there. The tank is very healthy and requires very little work to keep it up. My Zoa are doing great, I have a clam that it growing like crazy, trachi, hammers, anchors, bubble tips, crazy green star polps, a photo synthetic blue gorgonian that is growing very well, ultra green polyp leather, crazy fast growing Duncan, ultra welsos and last but not least a red planet and a patella doing just fine. The ONLY corals I have had a problem with are rainbow chalice. No idea why but I suspect the way I have the lights blended is the culprit. The ONLY complaint I have is that I should have purchased one more light for better coverage and I may do so shortly.
So I guess it comes down to preference.
The MHs do a wonderful job of growing SPS and pretty much anything else I put in the tank. I love the look of radiums and of course I have the T5s and Reefbrites for late night viewing. I don't care a bit about the cost of the bulbs, electricity or the chiller kicking on from time to time.
The LEDs do a wonderful job of growing everything with the exception of perhaps my rainbow chalice but again that may be how I have the lights setup. I love the color the lights produce and its fun fiddling with them. The MHs seem to grow SPS better based on friends tanks but I'm not sure that's not a husbandry issue as I have seen some tanks in person such as one of the displays at
WWC that has 3 Radions on it that is growing the heck out of SPS.
Also if your looking to switch maybe a T5 / Leds solution would be worth looking at. The Giesemann Aurora is one crazy nice fixture and grows the crap out of SPS.
Kinda long here - hope it was helpful.