Metal Halide Question?

I have a 90 gallon reef tank with a 500 watt metal halide system 2x250. I am currently using 14K lights double ended HQI. I have a combination of SPS, LPS, Leather corals, polyps, mushrooms, and anemones (both bubble tip and a magnificent). What would be a good light to get? Should I stick with the 14K or go with 13k, 15k, or 20k and what is the major difference between between them?