I think you mean pseudonimity. There should be no authentication in anonimity.

> Something you know (as in password) is always theoretically more secure than something you are (your physical characteristics).

If you authenticate, you can use methods that are more or less secure. If you use a password method, and use a secret as password, it will be more secure than if you use a biometric as password.

But the security of the authentication method will bear no weight on the "pseudonimization" - meaning on the difficulty of linking the authenticated identity with your real, legal identity.