I've been here for a few months and have been researching Prosites and WHMCS MU Provisioning.
I would just like to share what I've gathered thus far @wpmudev team feel correct me at any point.
This plugin main purpose is to allow the Provisioning/reselling of subsites on one WP multisite installation.
WHMCS MU Provisioning:
This plugin main purpose is to integrate with WHMCS billing and support application to enable Provisioning/reselling of individual WP installs or WP multisite installs on a server.
With Prosites you can offer multiple websites/sites on the same WordPress Install similar to wordpress.com
However with WHMCS MU Provisioning you offer multiple WordPress installs on the same server similar Zippykid.com
Prosites -> Managed WordPress websites/sites
WHMCS MU Provisioning -> Managed WordPress Hosting
Hope this helps.