I have to develop a SCU (Service Class User) application for querying PACS and retrieve the DICOM images.
Should I use C-GET or C-MOVE DICOM protocol to retrieve the images?
You can use either, but the preferred is C-Move.
There are two reasons for this.
As @NathanWheeler said in comments:
In security terms, a C-GET is as secure as retrieving your email... anyone can do it from anywhere if they have your username and password. A C-MOVE requires that the server recognizes YOUR COMPUTER as an authorized machine to receive information, so without explicit access, even if you have all the information to QUERY the server, you still can't retrieve anything. You CAN implement "promiscuous" mode, where if the
DestinationAE
isn't recognized, it sends the requested information back to the caller, but it's highly discouraged.