Subdirectories for subject relations

[5MMedia]

Dear fellow mozzers,

Im in doubt about a subject concerning the use of subdirectories to organize pages and content, but also the effect on how googlebot relates pages together through subdirectories.

The specifics:

Momentarily the website im working on a site which has a flat url structure for all pages. The website sells courses and eduction on beauty specific subjects. Imagine that there are multiple head categories like: pedicure. This head category has multiple subcourses which all have this structure (as do all pages within the site): http://www.domain.com/subject.

Now im wondering if it would be better to bring structure through adding subdirectories like http://www.domain.com/pedicure/pedicure-sub. I've read arguments on keeping your url short because this performs better. Nut on the other hand i think that google can use these subdirectories to determine the relation between head and sub.

The reason im asking this, is because the head category page is ranking better for keywords beloning to the subcourses. Would implementing subdirectories help to make the subpages rank better for their specific keywords?

Thx