ZPanel was written as an open-source cPanel alternative and is for users who do traditional shared hosting and reselling.
ServerPilot focuses on security to provide the best hosting control panel for developers and businesses running their PHP applications on cloud servers.
In 2014, hosting company Hostwinds purchased ZPanel.
If you want to run fast PHP applications and consider security and support critical to your business, then ServerPilot is the option for you.
If you need to host email, ZPanel (or other control panels) might be your better option. However, Google Apps or Rackspace Hosted Email both work excellently with ServerPilot to take care of your email needs.