14w per gallon seams like a lot--until you figure the tank may be 48" deep.
6w per gallon in a 12" deep tank is a lot more light than 6w per gallon in a 24" deep tank.
And the kelvin ratings on the bulbs too. Do you want super brilliant lights--like noonday sun? Then you need 5000k bulbs. They look washed out and white, but thats the spectrum some corals need. Or maybe you want a tank with a little more blue in it? Then your using 14K and 20K bulbs. But, those higher K rating bulbs aren't as bright.
It's all a compromise and totally dependant on a hundred factors that are unique to your tank.