In my opinion, you can roughly equate LEDs with MH. I believe a 2:1 ratio works, that means a 250 watt MH is roughly the equivalent of a 130 watt LED array. Some have suggested the ratio favors LED even more.
Again, in my opinion the factors that matter are total wattage and color mixture of the LEDs, more than the individual power of each LED.
Companies make all sorts of adjustments to their emitters, so wattage isn't the total story sometimes as the lensing can make a difference in how deep the PAR penetrates. So there is some factors there that could control how much light penetrates to the bottom, but at the cost of how many fixtures you need to cover the breadth of your tank.
Also, the electronics and emitters can be glued into the array, which isn't a good idea. They must also not be wired in a way that if one emitter blows the whole panel doesn't go out like a cheap string of Christmas lights.
Do your research on the Internet. I have heard good, but mostly bad reviews on the Cree emitters. Having a fixture with 3-7 watt emitters has the advantage of a smaller footprint, but not as small as you might think as the additional heat generated by these powerful emitters makes a lot of heat the heat sink has to conduct away. So they spread them out more. They don't project hardly any heat at all into the water (huge advantage) but they are like a transistor, they can get locally very hot and burn out prematurely. Most fixtures have a computer fan to vent heat, or very large cooling fins and heavy metal cases to dump heat.
As I stated earlier, I use larger panels with loads of emitters. They are a factor of half the cost of the higher output emitters and frankly, watts are watts and PAR is PAR. I would like to see a study where the high output LEDs were compared to equal wattage of smaller LEDs and their affect on SPS coral propagation. I will be looking into this for myself soon. As I stated, I am very satisfied with the larger panels of hundreds of 1 or 2 watt LEDs over the last 18 months. I have to cut my soft coral back weekly. LPS grows fine and my acropora's and even elkhorn is growing fine.
2/3 should be 10,000k white and the rest should be blue in any event.
