Should I use a sub-domain or folder for new blogs?

Hi there.

We are using Pro Sites to build a WPMU Network and we can't really decide if we're going with Sub-Domains for each blog, or just a folder.

What would you recommend as pro's and con's? Please keep in mind most of the blogs will have a primary domain pointed to them, even tho we still need a sub-domain or folder.

Thanks for your insights!