I am planning on creating a site with reviews and descriptons of various products that aren't related to each other. I will have the index page showing that this is a product review site and then from there have sub-domains for each product, acting as its own site (the index page will link to a middle page that will show the links of all the product subdomains so that the user can click on). Each sub-domain will have a few pages about the product and will have adsense on them. A couple of questions about this plan: 1) Should I have sub-directories instead of sub-domains? 2) Will it be harder to SEO a sub-directory or a sub-domain? 3) Is this a good plan overall and what would you do to correct it?
In your case, I think sub-directories are much more appropriate. Having a 2 page "main" domain site and and a handful of pages for each sub-domain will cause you issues in the long rung. Your site will never be seen as any type of authoritative site since this is based on your site having many inbound links to MANY different pages on your site. Both the main domain and the sub-domain only have a few pages each, so the number of inbound links will likely be spread thin across each of the sub-domains. And there aren't "many" pages on your domain or any of your subdomains. Also I typically use sub-domains for separating out different types of functionality on my site. For example, webmail.example.com, tools.example.com, secure.example.com, etc. Some larger sites are successful at having many sub-domains like CitySearch, but they have TONS of content for each sub-domain, even for a sub-domain for a single city. Also the maintenance effort for creating new subdomains for every review is going to get very old after a while. No... Same rules basically apply to both. But I think subdirectories are easier to deal with, especially if you are using a content management system. It's much easier to have your templates generate Breadcrumbs for instance in a CMS if the pages are all in the same site (as subdirectories) than it would be to do so if they were spread over essentially many sites (sub-domains). With subdirectories you can simply code the template to build the breadcrumb in reverse by taking the current page, getting it's parent and linking to it, then getting the parent's parent and linking to it, etc. til you hit the home page. As I mentioned above it will be much easier to build domain authority using subdirectories than it would be if using sub-domains. Same goes for domain trust. Also once your domain has established authority and trust, ALL pages on that domain inherit and benefit from the domain's authority and trust. Your sub-domains, however, will NOT inherit the main domain's authority and trust because they are viewed as separate sites and will therefore have to build their own authority and trust starting from scratch each time you create a new sub-domain. These are more advantages to going the subdirectory route. IMO the only "real" advantage or pro to using sub-domains is that IF (and only if) you had 2 pages on the main domain AND one or two pages on the subdomain that rank well for the same term, now you could possibly get a main listing and indented listing at Google for both the main and sub-domain. Normally Google will only show 2 listings max in the SERP for your domain. But since they typically view sub-domains as a different domain, you can sometimes get 3 or 4 listings out of 10 instead of just 1 or 2. I don't think so. I think it's a bad user experience personally... bad from a maintenance perspective... bad from an SEO perspective... just bad all around. There are a few sites out there that are good at using subdomains like this (about.com, citysearch.com, etc.) but they have BUTTLOADS of content they are managing with thousands of writers/contributors. IMO Using sub-directories is definitely the way to go for this particular application.
I really appreciate the time you took to reply to my answer. What you said was an eye-opener and everything makes sense in what you said. Thanks again for your very valuable answer.