i only have one pc, directly connected to the ISP switch, if you use a router and have a LAN at the home, then force 100 Mbps full.
If you only have one PC connected to the modem, use half duplex for Cable, and link speed depend on the modem port, i suggest to start with 10 Mbps half duplex and then compare with 100 Half duplex.
If you have dsl, try both half and full, some dsl connections is full duplex
VanBuren