When is a Spell considered to be "Cast"?
Neither. A spell is cast after its costs are payed, before anyone has a chance to respond to it, before it resolves. From the rules:


  601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. 


So yes, if you cast Silence, it will be too late to stop any spells that are already on the stack. This is also important for effects that say "whenever you cast a spell", for example.