first of all, because you have more incoming links to site A, so you should TRY to forward the Site B with 301 permanent redirect to site A, because it will cause a drop in traffic as the search engines will have to re-evaluate your
PR.
Secondly, if you implement this, i would like you to retain the directory structure, or setup 301 permanent redirect for all the current files on site A to site B so search engines do not encounter page not found error for any of your pages, it would be alot of work if you have a large website, but it is the right way to go. also when people click on the SERP pages on your links, they will also not encounter the page not found error. You can setup the error handling documents to handle this.
Thirdly, I would recommend you to keep both websites separate and redesign your website in a way that you have reviews of products on Site A and the visitor is presented an option to buy that product, and when he click on that link, he is taken to site A.