Release 2.0 for SAM4 electronics and 1.21 for SAM3X electronics
Version 2.0 for SAM4-based electronics is the first full release of RepRapFirmware to use a real time operating system. It also brings a number of new features and bug fixes. See https://github.com/dc42/RepRapFirmware/blob/dev/WHATS_NEW.md for the upgrade notes full change list.
Version 1.21.1 is a maintenance release for SAM3X-based electronics with the same bug fixes (where applicable) and most of the new features.
The DuetWiFiServer.bin file, Windows device driver files (DuetDriverFiles.zip) and iap files in this release are unchanged from the ones in release 1.21. So there is no need to upgrade them if you are currently using release 1.21.