There are several advanced features in Nuendo that you may have no need for. I suggest that you check the feature list of Cubase, and see what, if anything, is missing from your ideal program. Then make a choice.
As far as OS, Windows is by far the better choice, not only because it is more efficient with Cubase, but if you intend to use PLAY, you really don’t want to be running it on OSX, unless you have no other option.
DG