Shannon,
The difference is that the MCX can terminate voice or data circuits and can do TDM cross connects between ports. The MC is a data card.
Both the MC and MCX have built in CSU/DSUs.
http://www.cisco.com/en/US/products/hw/modules/ps2033/prod_bulletin09186a008007d45a.html
http://www.cisco.com/en/US/products/hw/modules/ps2033/products_data_sheet09186a0080091c34.html
The MIX-enabled T1/E1 port adapters provide the ability to integrate voice and data on the same T1/E1 interface without the need for an external time-division multiplexing (TDM) device. With the "drop and insert" capability, DS0-level grooming is supported, thus providing cost-effective WAN access. Ports can be configured as voice, data, or both, depending on the application requirements.
Please rate helpful posts.
Dave