Deciding to keep all of your content on the same domain or having them be split up onto separate sub-domains is a legitimate question to ask yourself if you have multiple domains that possess similar content in relation to at least one ore more separate sub-domains.
Russ Jones, the author of the article entitled, Solving the Sub-Domain Equation: Predicting Traffic and Value when Merging Sub-Domains, has been in the SEO game for a number of years now. He has had to deal with sub-folder vs sub-domain for almost as long as he’s been doing SEO. In his article, Russ wrote about the problem of whether you should keep all of your content on separate sub-domains, or stored completely on one single domain.
This may sound trivial, but it can bring up interesting concerns, such as if you will lose or gain traffic in the process of switching from one domain to many, or vice versa.
Here is the following article that you can read that talks about this problem that some site owners might face: