What Cloud Storage Metering to Use to Track Individual User's Use: Mezeo, Vembu or Other?

Question

We want to offer cloud storage services as part of a portal and want to meter the usage of each user and bill them for that usage.

Basic features needed are:

1. Meter actual usage per user account (and either provide invoice per user or allow easy API tools to pull such data into our reporting/billing program)
2. Backup management
3. Synch files
4. View files over Browser
5. Back up different files including Outlook, Exchange, MySQL

Which tool -- Mezeo, Vembu, other -- would you recommend and why?