For C# dev, it depends how hardcore you're going to be getting. Ultimately, you normally going to be scripting, not programming. getting familiar with the Unity3D API is of more use than learning high level features of C#. For instance, while C# is object oriented, >90% of the things you write will inherit from MonoBehaviour, and most of the rest are just fancy structs.
And I code on an iMac -- I use Unity natively, and the version of MonoDevelop it ships with. It works nicely!