In terms of free traffic methods e.g. article marketing, SEO, web 2.0 traffic etc what do you find brings more sales/conversions and visitors in the long run (4-6 months) a one page lander with a simple lead capture form or a properly built up authority site with a lead capture set in, product reviews for different products in that niche, a blog, perhaps some tutorials etc?
An authority site is the best option for long run. This will build the trust between you and the visitor. However, one page site is good to promote a single product for quick cash.
I'd say authority sites. One page landers are good for PPC and banner ads, but larger sites will increase trust with your audience and gain better search rankings. I usually have what looks like a one page lander for articles and PPC, but include a link at the bottom to the rest of the site. Once they get into the site, it looks more like a blog or article directory.