Are you actually comparing Darwin vs Linux, or OSX vs GNU?

Proprietary software has many advantages over Free - heavier corporate investment creates more consistent and polished results. But that doesn't imply that it's better for all purposes, or that a new project is better off choosing a license that makes easy to incorporate into proprietary products.

Darwin is an OS (the kernel is called XNU), so I was comparing it to Linux-based OS's. Darwin is the basis of both OS X and iOS. I agree that whether or not proprietary or copyleft is better or worse for a given situation is hard to say. A BSD license simply leaves the door open.