Nema will share waypoints, but only those created while linked. ie if you create a waypoint at the console graph and bow graph is tuned off, it willnot show when you power up the bow graph.
Nema will control some funtions on other graphs, ie if you adjust the brightness on console graph if enabled, nema will adjust bow graph as well, ethernet will not.
Nema will take data from compasses, control sonic hub, external temp sensors etc etc. Ethernet will not.
Ethernet will share maps cards with gen 2 and up
Ethernet will update all waypoints on each unit when powered up.
Ethernet will share sonar, side and down with other graphs
Those are the basics, both have lots of other features too, to have a fully operational system to all of it's capabilities you really need both...