Subdirectory URLs

[Visually]

If I have category pages for my site; is it better to use http://example.com/category/_category_ or just http://example.com/_category_?

Also, I'm creating a new section of the site; a resource center. Should the URLs of the pages in the resource center be http://example.com/learn/_page_ or just http://example.com/_page_

What are the reasons for the better choice?

[Danostatic]

I have actually had hands on experience with this.  You will likely read almost everywhere that the shorter, the better but it wasn't the case for one of our clients.  We had a client who was ranking pretty high for a very competitive word.  We began going through an on-site SEO audit and clean up issues such a tags, meta, alt, etc.  We also decided to shorten all of the URL's to http://example.com/_page and set up 301's to point to the new url.  It immediately lost all rankings even after a very lengthy wait of a de/re index.  Their rankings came back  quickly after we moved it back to long tail._

I doubt this will be the case for all scenarios, but it was better to keep the category in this case in the URL.