There's no official definitive answer for this question but here's my best:
The rule of thumb for SEO and specifically Google SEO is to build your site with structure that is most usable, logical, and friendly for the user.
Subdomain vs Subdirectory are just two different methods of organizing your content. The primary difference between the two is how they are managed. You can manage subdomains separately from your main domain using DNS or a CNAME. This means you can create usedcars.yourdomain.com today and at anytime break that out into www.used-car-website.com using DNS should you choose to.
The other benefit I can think of is using subdomains to control Page 1 of your brands search results. Unless you have extremely high authority, Google will only display 1 - 2 results in searches from each hostname/subdomain. By having substantial content which differs from your main site broken out into subdomains you gain the benefit of potential increased Page One SERPs. If the content is distinct and unrelated to the subject matter of your MD, Google may give you love for making life easier on the user. Google uses subdomains for their distinct functions; videos.google.com, news.google.com, etc.
Checkered Flag is a good example of GPOM using sub domains.
Something else to consider is that if you use subdomains but don't interlink the SD site to your MD site they will be considered two separate sites by Google.
I know this isn't clear cut but you'll find in the world of SEO answers rarely are.
Hope this helps. 
