
If you have a Parallels Desktop subscription, then your Parallels Access subscription starts right after you complete the online purchase or register your license under your Parallels account, if you purchased a boxed copy. The subscription starts at the moment of registration.

If you have a Parallels Desktop perpetual license, you need to register it under your Parallels account.

The main advantage however is that Virtual Box is Open Source and cross-platform. can easily be run and maintained from the command line (or remotely through SSH).

There is quite a high lever of integration to the Ubuntu host OS:

This will not only give you up to date bug fixes but also enables USB2.0 and RDP support. We may install a OSE edition from Ubuntu Software Center but to my experience it is better to run the most recent version from the Oracle Repository (see this answer for installation). A quite stable actively developed alternative to Parallels Desktop for Ubuntu is Virtual Box.
