What bindings and security configurations are best for an environment with Windows and Mac Computers?
BasicHttpBinding will probably have the greatest reach. Just about every platform supports it, since the data is simply sent as an HTTP POST to a specific Uri -- any platform which has a web browser can use this binding.
You can do an HTTPs binding with BasicHttpBinding as well -- so you can still secure your data. You can also use security like TransportWithMessageCredential to provide username/password validation.