I have run Metal Halides for over four years, ever since I started reefing. I have tried numerous different bulbs and colors and my opinion is that the 14K bulbs from Hamilton have the most aesthetically pleasing color and at the same time provide good growth rates for corals. I have found that 10K bulbs possess better growth rates than 14K bulbs but are not as pleasing to the eye being on the more yellow side of white. I have always run my Metal Halides with VHO Actinic Supplementation. Right now I have a combination of 10K and 14K bulbs over my tank supplemented with VHO Actinics.
If you are runnning All actinic VHO bulbs over your tank now (half Super Actinic and half Actinic White) and you are planning to take out the two actinic white bulbs and replace them with two 10K Metal Halide Bulbs, you are going to be surprised in two ways. First you are going to be amazed about how much brighter your tank is. Second, you will be stunned by how much whiter/yellow your tank is. You have all actinic bulbs over your tank now giving your tank a nice deep blue look to it. Therefore, I think you should get 14K bulbs when you switch and not 10K. You can always get 10K bulbs later on down the road.
I reccommend Hamilton and UShio as the brands that were the best. I am trying 10K XM bulbs right now, but one of them has already burnt out on me after 4-5 months. I have never had a Hamilton or Ushio bulb burn out on me yet, and I replace them on a yearly basis.
I replaced the burnt out 10K XM bulb with a 6 month old 10K Ushio. I left the other XM bulbs in the fixture so I could compare the two bulbs. I cannot tell any difference in brightness or color between the two bulbs. Both have around 6-7 months burn time on them. Most people say that the XM bulbs have higher par and a more bright-ice-white color compared to ushio bulbs that are said to be more yellow and not as bright. But, I really can't tell the difference. The two 14K bulbs I have running in another fixture over the same tank, are about 75-80 percent the brightness of the 10K bulbs, but they are so much more blue. There is no tinge of yellow, and the corals seem to flouresce more under them than corals located more under the 10K bulbs.
Nice looking tank by the way. You had better get good at fragging if you switch to Metal Halide, because your corals will grow quite a bit faster once they get used to the new brighter lighting.
HTH
Chris