Different versions of websites are similar to differentiating between better ppc ads. If there are links at the bottom connecting to a company’s facebook, twitter, etc., then those links will be more effective with CTRs because more information is readily available. You could also look into analytics and find out which parts of a site have more clicks than others. It is extremely useful to compare the CTRs of those similar/different parts when analyzing effectiveness of a webpage. Also on a side note, I think there is some sort of statistic saying people’s eyes enjoy looking more at dark backgrounds and white fonts rather than the opposite. More companies should look into the effects of such eye preferences, and hold other things constant, when gauging the effectiveness of a website design. I also think less wordy, simple things are easier at grabbing attention, but it all depends on the content and call to action.