EtherTalk in MacTCP is for sending AppleTalk packets with encapsulated TCP.
Use this choice for networks that must go through a DDP/IP router. The
Ethertalk setting creates the Datagram Delivery Packets that the router
expects.
Your network apparently has direct access to the TCP host, which lets you
use the straight packets (Ethernet choice). The way to look at this
setting is that it's one or the other, not both.