Permissions when crossing over from Windows to Mac
I've got a nearly identical setup... a Windows 8.1 box using IIS and an OSX laptop using MAMP with Craft installs that are sync'd with Dropbox between them.

If I recall correctly, the only thing I did was give "Everyone" full recursive access to things like craft/storage on Windows and chmod -R 777 craft/storage on the OSX box.

Haven't ran into any permission issues.

I will warn you not to use symlinks in that setup, however, as Dropbox doesn't know how to resolve those cross platform.