This question already has an answer here:
- “It isn't” vs. “it's not” 4 answers
Everyone agrees that:
Semantically "It's not" and "It isn't" mean the same thing: "It is not".
No doubt here. See: http://answers.yahoo.com/question/index?qid=20110704115859AAmQmxZ
In general: which one is more common and what examples where it makes difference come to your mind?