If you choose C++ language, I would definitely recommend Qt for that. Your application can be deployed under Windows and Mac as requested in the OP, but also Linux, and now, with the latest version on smartphones using iOs, Windows RT and Android.
It's well documented and very active on the web (including on SO).
The only negative point I see is Qt creator tool which, IMHO, is less user friendly than historical tools (like Visual Studio for instance ), but you are not actually forced to use it as an IDE for Qt development.