Spanning's web site has an answer for your question, along with a comparison of features in Spanning Backup and Vault. One of the sections on that page is titled: Do You Want eDiscovery or Recovery?
As JP mentioned, Vault is limited to Drive, Gmail, and Hangouts. Spanning backs up Gmail, Drive, Calendar, Contacts and Sites. Google Apps will now allow you to restore deleted contacts and events, but only for 30 days.
Users can restore their own data from Spanning. (Some other backup services allow that, too.)
You can run trials of Spanning and Backupify at the same time, because both run in the cloud - none of your bandwidth is used during the backup process.
When you compare services, ignore the sticker prices. Fees for GfE domains vary. Get a quote from each company based on your number of students and employees.