Multisite. Sub-domains or sub-directories?

Okay. First off, you guys are awesome support. Really. So ... i'm building a multisite network that's meant to incorporate eCommerce, a wiki, buddypress. Very graphical, with a membership site and sitewide branding.

I want to allow people to have their own blogs within the site or commerce spots. What are the pro's and con's to sub-domains and sub-directories? (including the opinion on which people prefer to work with as you've encountered)