What's different between M2US50 vs M2SD50?

What's different between M2US50 vs M2SD50? Laird built the M2SD50 board and the M2US50 board differently: the M2US50 supports only the USB interface on the M2 interface, and the M2SD50 supports only the SDIO interface on the M2 interface. An M2US50 module is configured to only support USB.   Note, Laird does NOT support USB on WindowsCE.