I was looking for an article/post/blog/something that mentioned the difference between the Modem and Printer ports, saying something technical about one versus the other. I can't remember where it was and it's not in my bookmarks or history. Something to do about xfering data without as much interference from the CPU or thereabouts. Maybe it was a reference to something described in Inside Macintosh...dunno.

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You may have seen this post: viewtopic.php?f=7&t=21226

Also this IIgs technote mentions the interrupt A as being special: http://www.umich.edu/~archive/apple2/technotes/tn/iigs/TN.IIGS.018

In practice, there is no difference, just two serial ports hanging off of the Zilog SCC. Apple just switches the communication mode to SDLC to whatever port Localtalk is being used on. The Etherwave likely uses the same transfer mode that Localtalk networking uses (230kbps) to achieve that breakneck speed. Out of the box the SCC is capable of 115kbps with standard RS-232 signaling, the real question is if the host machine can handle it as most aren't fast enough.

It really doesn't matter which port you use. Apple's software gives you the option to use either the printer port or the modem port for Localtalk. The same port flexibility exists on the slower Apple IIgs too.