Now I've heard (I could have be wrong) that the higher the "K" the blueer thr bulb, but with that the draw back is that you have to get the bulbs changed more often compared to the 10K (that last longer)!
Well, yes you are correct. I personally think a 10k bulb is fine. But I might would go with a 14k bulb. But not 20k, unless you wanted to mix one in amongst a couple other 14k's or something. Of his two choices, I would go 14k.
I would think the 14k would be better light for your corals. I doubt I would run a 20k. I don't know what the consequences would be though. I would however think your corals would appreciate the 14k bulb way more...
I really think the 20k would be better used if you had 3 or more bulbs and wanted to mix in a 20k for a more blue tint. JMO though.