RaiDrive is one of the most popular ways to get cloud and FTP storage into Windows Explorer. It is free for the common services, widely used, and does one thing well: it makes remote storage look like a local drive. So why compare it to FTPie at all? Because the two solve the same underlying problem - "I need to work with my remote files on Windows" - in very different ways.
The short version: RaiDrive mounts; FTPie manages. If you want every app on your PC to see your cloud as drive Z:, RaiDrive is built for exactly that. If you want a dedicated workspace to browse, transfer, edit, back up and share across all your storage, that is FTPie.