Updating project between Windows and Mac

I made a project on a Windows computer and transferred it to a MacBook.
Now I have two versions of it.
I’ve done some more work on the Windows version and would like to update the MacBook version.

Do I need to simply copy and paste the Assets folder or is there another technique?

Use a Version control system.

http://answers.unity3d.com/questions/309779/subversion-and-unity-projects.html