What is source download?
Download from the server of the site who is providing the document. For eg. in case of Practice Manual it is ICAI server, as ICAI provides practice manual.
What is mirror download?
Download an exact copy of original file from Cacharya server. It would be a backup, if source server is down or is not responding. For eg. if ICAI file server is not responding and you want practice manual then you don't have to wait till it responds, you can still download it. Also file names are more meaningful than ICAI's, download a file from both server and see with what name it saves.
NOTE: Content wise files are same, you can download from either, no difference.