250w MH on a 15g-> what bulb?

[Soccer]

Hey all,

I am currrently transfering everything from my 10g to a 15g and in the process I am going to install MH lighting. I have a standard 250w MH ballast aound and I was wondering what bulb to run on it. I have limitations since the ballast is probe start but I am leaning towards a 20k XM bulb. Also the 15k XM is something I am looking at, but I have heard that the more blue in the spectrum the less intense the light will be on corals in the tank. So I was thinking that the 20K would give a nice blue/purple colour which won't be as intense as a 10 or 15k since 250w of MH is a lot over a 15g. Does anyone have any pics comparing the 15k vs 20k XM? Are the XM bulbs my best choice?

All opinions/info are appreciated thanks