Did you know that if you purchase wither AutoCAD 2013 for Windows or AutoCAD for Mac 2013 you can activate both using the same license? Yes you can use same license to activate both (2 machines in 12 month period). The same goes for AutoCAD Product Suites.
You use both a Mac and Windows machine and want to use native AutoCAD versions on both platforms. You purchased AutoCAD 2013 for Windows. Do you need to also purchase AutoCAD for Mac 2013, the answer is no. Just activate the AutoCAD for Mac using the serial from the AutoCAD 2013 for Windows. You can activate 2 machines in a twelve month period. So you could purchase AutoCAD for Windows or the AutoCAD Design Suite then download the trial AutoCAD for Mac and activate it. Both Windows and Mac versions can also be licensed on a network license server.
The DWG files are exactly the same. Many of the customizations such as LISP, Scripts, Sheet Sets (Project Mgr. Palette on Mac) are compatible so you can work in a mixed environment.
So if you have users in your company that prefer a native Mac version of AutoCAD, they can easily do so and use the AutoCAD for Windows license and/or server.