Many thanks, folks! You are very responsive and helpful.
We are a mostly Apple house for my wife's benefit. I'm retired IT, and decided that since the Intel Mac Minis will most likely be discontinued next Tuesday I would buy the biggest, fastest one Apple makes to hopefully future-proof. With 2TB of space, i think I'll just divide it in two and give a terabyte to each OS. Plex runs great on either, and it is comforting to know that Channels DVR does, too. Plex works great on both OSes as well.
I run cTivo on the Mac side to archive our TiVo recordings to iTunes and also Plex format, but they can easily be copied to the Windows Plex install.
Not to mention that if I ever run out of space an external drive running off a Thunderbolt port will do nicely.
Another reason to buy the mini is, it can run 24/7 whereas my Mac Air would give up the ghost.