Depends a bit on how you want it to function. I prefer redundancy, and dislike having someone at one station changing the display on the others. E.g. if I'm working on a chart or
radar down below, it shouldn't mess with the helmsman's display and
vice versa. Of course, I'm also paranoid and consider VNC to add an unnecessary link of dubious
reliability.
Thus I'd prefer separate systems that are able to synchronize data between them, such that if I create routes, add chart notes, etc on one system they are cascaded to the others.
This can get costly if you have to pay for
charts though. Which reminds me, the last boat I was on had three
B&G plotters, as I believe all can use a
single chart card regardless of which of them has it inserted. This, along with data synchronization, would be useful for OpenCPN.