none of them? even if you would offer a webinterface that is able to connect to data stored at a certain and one provider would be enough (amazon or google) for example… this would solve the windows issue completely, as it would be possible to access the data from the macos app and also via your webinterface (the development is already done as the webserver already exists). you would not be responsible to run a datacenter. you would not have to develop a windows version. you would not have to develop a new software (the webserver already exists, you just redirect it to a cloud service instead of a locally installed server by the user)… what do you think about it? 