When you create a FinBIF account, you can safely choose which ever method you want. You can use one of the authentication methods or create a new username and password (FinBIF account). You can always later associate other authentication methods to your account. Regardless of the chosen authentication method, FinBIF will ask you to confirm your email address. You can choose which email address you want to use as a contact email. You can only have one FinBIF account for each confirmed email address.
Try to avoid creating many accounts for yourself, unless you especially want to use different accounts.
Some FinBIF services (such as services for public authorities) require a stronger authentication method. These requirements are told on the login page of those services.