if your wanting to actually light a tank using LEDs seems the trend is to use a 50/50 split of cree XRE royal blue and white to create a 14k look.
http://www.ledsupply.com/ for those that are DIY minded they are quite powerful especially when coupled with a focusing lense they produce very very high PAR numbers.
tighter the beam the higher the comparison to MHs.
if you want to look at building your own nano-reef.com in the lighting section has a ton of articles and threads about builds and how to do it. so far alot of success for total tank lighting.
also you get the shimmer effect of MHs some say even more so then a MH due to LEDs shimmering already.