Use Factory Design Pattern when
- a class cannot anticipate the class of objects it must 
create
- a class wants its subclasses to specify the objects it 
creates
- classes delegate the responsibility to one of several 
helper subclasses and you want to knowledge of which helper 
subclass is the delegate.

Use Singleton Pattern when
- there must be one instance of a class and it must be 
accessible to clients from well known access point.
-the sole instance should be extensible by subclassing, and 
client should be able to use an extended instance without 
modifying their code.