Which is best domain to use for SEO?

Which is best domain to use for SEO?

I've got a bit of a dilemma. I have two domains - let's try a primary domain that uses a keyword like car_com - excellent domain. Now let's say I want to put up a site that has a directory of all the car washes in the US. I also happen to own carwash_com too. There are two questions - one is of branding and one is of utility of having customers on your site. Choices become as follows:

(a) car.com/wash
(b) carwash.com

So which would be optimal to use? Option A allows me to have tons of extra content on the car.com domain and it will be indexed with Google as belonging to car.com. Using option B has keywords directly in the domain but does not provide the content database to be added to the car.com domain and there also might be the issue of branding, e.g. promoting the car.com domain as often as possible. Use both? Sure, but only one can use the unique car wash database or else you would get penalized for duplicate content.

So I would really enjoy hearing opinions on what you think would be the best course of action, especially from an SEO perspective. Thank you!