I think either/or would be good - once you decide to go with one though, the important thing is - is to stick with it.  Also make sure that your code, HTML, and more reflect the choice that you made.  For example, you will want to make sure you use sub.domain.com as your URL for that site always to make sure search engine robots don't crawl that path and the domain.com/sub/ path and then lower your ranking because of duplicate content.

Yes, definitely additional administrative issues come with subdomains.  They're a piece of cake to set up in Lunar Pages CP, but managing separate sites can be a hassle. 

Subdomains are a really good feature when you want to isolate software applications, like forums or e-commerce systems.  Keeping that code isolated in a subdomain and using a separate database helps you manage software more easily and with less risk of an error crashing an entire website. 

And someone correct me if I'm wrong, but as of a couple of years ago using subdomains was really beneficial in search engine marketing.  SE's considered (and certainly still do to some degree) a subdomain as a completely new and separate web site.  Just make sure there's enough content there to justify it.

If you're going to use it for separating content as you suggested, and not for a specific software system, I'd start with the easier domain.com/cars, and if you develop enough content there to justify using a subdomain, turn that folder into a subdomain and use 301 redirects to point inbound traffic from the domain.com/cars location to the to cars.domain.com location.

Jerry