Should I develop my client to use RSLinx Classic's OPC or C-API interface to communicate with my processors?

Use the OPC interface for data acquisition applications. This interface was designed to hide the routing complexity introduced by the ControlLogix Gateway by using a topic definition to specify the target device. Also, data requests are optimized to reduce network traffic by combining multiple request in a single packet. This is significant when communicating with a ControlLogix processor. Data acquisition through the C-API is not optimized in RSLinx Classic, but instead must be done in the client application.