given my limited SW knowledge right now, I would let someone else chime in before trusting my answer...
but, my understanding of the different K's is that it is just color preference. The 12K will be slightly more blue than the 10K. If it looks too blue, then you will want to compensate it with another bulb being less blue (<12K and not actinic).
What I am not sure on is if there is something beneficial about 12K over 10K for corals.