Unless you have some reason to have the data in an FX70 and the import the points again via the Niagara protocol to the AX workstation I would bring them directly into the AX supervisor (or server as you call it). This will be faster getting the data points and they don't have to exist twice.
In the end it depends on what your are trying to accomplish.
I guess the only reason is price. We already have a dedicated jace and a supervisor. Question is ... Is it cheaper to get the jace licensed for bacnet IP? I dont think there is a limit of points. Or do I license the Supervisor? The Supiervisor Bacnet IP point pricing is in blocks of 500.