This is an expansive subject to say the least. But, it really depends on what you like. 10k's are more yellowish, white. 14k's are bluish and 20k's are very blue, but give you excellent color IMO. The trade off is that you supposedly get less growth with a 20k than you would with a 14k and less still than a 10k.
If you're trying to grow out an get some better color I think 14k's look great, but that's my taste.
I used to run 10's. Had 14's for a year and liked the look, but didn't get as much growth. I just switched back to 10's as an experiment. Color is much different, but I have seen a boom in growth. (Also boom in green algae on glass).
Ultimately 14k's are for me. What's for you????
