True the choice is yours, but I still do not recommend subdomains when the comment is similar and can be directly related to each other. There are many benefits outside of the “subdomain” issue, such as: depth of content, immediate authority granted from the main domain, and ease of ranking pages of the site. Most SEO experts out there will agree that sub-directories are a better way to go, but there is always an exception to the rule.
From Matt Cutts – http://www.mattcutts.com/blog/subdomains-and-subdirectories/ “For several years Google has used something called “host crowding,” which means that Google will show up to two results from each hostname/subdomain of a domain name. That approach works very well to show 1-2 results from a subdomain, but we did hear complaints that for some types of searches (e.g. esoteric or long-tail searches), Google could return a search page with lots of results all from one domain. In the last few weeks we changed our algorithms to make that less likely to happen in the future.”
Google stated at PubCon a couple years ago that they treat subdomains as sub-directpries for rankings, but since there is additional benefit from having them as sub-directories … I still strongly recommend that you use sub-directories over subdomains.
http://www.vbseo.com/f104/matt-cutts-subdomains-subdirectories-19100/
http://www.ditii.com/2007/12/07/google-matt-cutts-sub-domains-to-be-treated-as-folders/
“A post by Tedster at WebmasterWorld, where Tedster said: News flash from Las Vegas PubCon. Matt Cutts informed us that Google will very soon begin treating subdomains and subdirectories the same in this fashion: there will be only 2 total urls from a domain in any set of search results, so no more getting 3, 4 or however many spots via subdomains. We didn’t get any more information than just that basic heads-up.”