10K is best for growth but it's yellow as heck, in my opinion.
I scoured the interwebs for my bulbs in January. Best deal I found was on Amazon from Hellolights. I checked Hellolights website and the bulbs were more than if I just got them on Amazon. I looked at Marine Depot, Premium Aquatics, DFS, as well as a bunch of smaller no-name places too.
I love 20K if you don't have actinic supplements. BUT if you have any actinics I'd go 14K.
I am really thinking of getting Dave to help me build a blue LED actinic fixture for my 120 and then going to 10-12K for my main bulbs. I will probably not like it though...after years of doing 20Ks. I use either XMs or Radiums. I can't ever remember which. I always have to ask Randy. One of them lasts only about 6 mo and the other doesn't. I use the type that last longer (Radiums, I think).