They are not same at all. Say you take a backup of R80.40 gateway and then clean install the machine, you can use that backup to restore the config back. Migrate export is more used to export the objects and rules from one management server and import them onto another that can be different software version (say if you migrate export R80.10 mgmt server using migrate tools from R80.40 clean install server, you can then use that export from R80.10 to import all those objects and rules into R80.40 box)
Andy