So what do you think it's more effective ? Writing about main product and point his pro points or compare main product to a 2nd one and emphase the strong points of the main product ? From what i saw till now head-to-head style brings in more sales for the main product. Ofc the basic rules stay the same: recommend products that work, solve a problem, make a connection (from a freebie http://www.imguru.biz/internet-marketing/affiliate-marketing/23-reviews-in-affiliate-marketing.html )
It all depends on what type of site you're setting up. If it's a review style site, then I would say reviewing 2-3 is better. People like to have choices and if they come to your site and only see 1 option, they might be left with a feeling of "I'll see what else is out there first before I make a decision." Think about it as if you went to the store for some new shoes or something, and they only had 1 pair of shoes in the entire store. You would probably leave and check out other stores to see if they had something else you liked better. (That might not be the best example, but you get my point.) However, if you built a blog where you tell a story of how you had a problem and you finally found that ONE solution that worked for you, then of course it's best to just have the one product listed. So it all depends on if you want your site to be setup as a review site, or more as a personal blog of your character on the site telling a story of how they found the best solution to their problem. That's my opinion anyway.