MSTCP is the name of TCP/IP protocol on Windows 9x/ME, but in context of WinpkFilter it is used to specify Microsoft TCP/IP module on Windows systems.
Sending packets to MSTCP means that packet will be indicated to TCP/IP stack from the name of the specified network interface (just like this packet would arrive to this network interfcae from the real network). By other words packet is injected into the upstream between network card and TCP/IP stack.
Sending packets to adapter means that packet will be sent directly to network interface from the name of TCP/IP protocol. By other words packet is injected into the downstream between TCP/IP and network card.