> What's the phylosophical difference between these for singletons?
IMHO, from a conceptual point of view, none of these selector is
appropriate for a Singleton. While they provide a global access point,
both #current and #default imply there might be some other instances
(not current or not default) of the same class.
Singleton's intent is: "Ensure a class only has one instance, and
provide a global point of access to it.".