If you are looking for coral growth and health go with the 10K and supplement with actinics, if you are going more for looks then go with a 15K or 20K.
I agree with changing the bulbs out every 8 to 12 months. When mine get to the 8 month mark I have a new set ready to change out. I notice that diatoms and cyano
become more and more pronounced and my corals don't open as much. I recently changed out my bulbs and switched back to 14K and the difference is amazing. My corals are just beautiful and I can tell a difference in growth week from week. (I take pictures every week of my corals)
I say better safe then sorry and change them out every 8 to 12 months. Over all it isn't that great of an expense. I would much rather see my tank healthy and happy then get a few extra months of use out of a set of bulbs.