It might be good to master at least 2 paid traffic/marketing methods (adwords/ppc, ezine advertising) AND 2 free traffic/marketing methods (e.g. seo, article marketing, social network marketing).
Of course, there is really no such thing as 100% free....you will pay for traffic, either paying with money, or with your time.
The question about which method or source works best for an affiliate product may not be meaningful because it also depends on several other factors i.e. your copywriting skills, your ability to market using ppc, your ability to market using articles, your ability and willingness to use paid marketing resources/services.
Cheerio