The discussion around Marketplace functionality began in an issue and is a currently a work in progress. There is a basic comparison (dated 2013 Dec) of the two solutions, which differ in their implementation approaches. No matter which path you take, marketplace functionality will require development expertise to implement correctly.
Maciej's Commerce Marketplace
Maciej's implementation is available as sandbox. This approach aims to extend Drupal Commerce's core functionality as much as possible and keep in line with what might be considered the "right way" to do things in Commerce. Maciej's thoughts and progress on Marketplace in Drupal Commerce are available in blog posts that discuss use cases and installation, payments, and existing functionality and configuration.