some time ago Google equalised subdomains and subfolders - officially... the differences are marginal but they exist and I will tell you a few which you can think about ... maybe this will help your for your decision.
In the first place Google regards a subdomain as a separate domain within such a theoretical scenario: you can get up to 2 listings within the serps for one keyword with one domain. Using a subdomain multiplies this possibility (I`ve seen examples where a single subdomain had 5 listings + 2 from the main domain)... so this is a factor which can have an effect on the traffic.
Another aspect are penalties. If you get a penalty on a subdomain, there is a very big chance that the main domain is not affected. By the way: this process is NOT reversible!
Furthermore, if you use sudomains it`s much more easier to control htaccess (e.g. mod rewrite)
Using subfolders is often much more easier to handle for webmasters but that can`t be seen as "across-the-board".
The main argument you will get to hear about using subfolders is that they get trust from the main website. There is no evidence that this is an advantage of subfolders. Subfolders and subdomains need links from other sides to establish their ranking!
So if the content you would like to separate has less in common with the content of the main domain you should think about the option to use subdomains.