Dear Dave, thank you for posting your questions here.
I would suggest that you get the download version for mac osx. You are right that the download is large and in fact you will need twice the disk space for the download and install i.e. 2x18Gb disk space. You can only get the addons for the download version. Basically future aerofly simulators on the mac osx platform will only be released as download versions, unlike the windows version which will continue to be available in DVD and download versions.
Yes, the aeroflyFS mac DVD and download versions are identical in graphics, flight physics, location map, etc but as I said, please get the download version. Hope this clears the air for you.