Yea that thread has some interesting ideas doesn't it? Well, here is my thinking on the topic of wattage vs. penetration to the substrate etc.... If you have a new tank and you are still setting up your fertilization and CO2 methods and you don't currently have any high light plants, then starting at the low end and going up is better than starting bright and then dealing with the headache that will inevitably create, IMO. The price of CFL bulbs is so reasonable that swapping them out for higher or lower wattages is trivial really.
Look, the bottom line is that we want beautiful tanks, with healthy inhabitants, right? We all know that light drives the need for all other requirements. Ferts, CO2, animals, algae (and it's control) are all determined first by the amount of light energy available. I personally have a high light, CO2 driven, EI dosed, hugely productive (like major trim every week) 36" long tank with (3) 23w CFLs on it. Right next to it, I have a 15 gallon Aqueon kit tank with the 18" fluorescent bulb it came with, no CO2, no ferts, and 5 fish in it. I cull my trimmings from the bigger tank and plant that tank with them. I get as many compliments on that tank as I do the larger one. No, I can't grow HG or other high light plants in it (I think - I haven't tried but someday I might!) but the stems and moss and ferns are healthy and growing. I'm just not in there weekly with scissors. So, I guess it comes down to what you really want to do. Sometimes I wonder whether I would be happier with a super low tech tank with darned near no maintenance other than water changes...
Just keep in mind that if you're not OCD with your tanks, that the higher amount of light you put over it the more time and energy it will REQUIRE daily and/or weekly.
(big breath) So, to answer your question! I would start with 13 watt 6500k, and go from there.
HTH - Cheers