"No, it isn't" versus "No, it is not"

(Grammar Questions I Couldn't Answer)

This question comes again from a colleague instead of from a student.

He was preparing a lesson for beginner students, in which they he was supposed to drill "Is it a...?" questions, and the answers "Yes, it is." and "No, it isn't".

He asked me if there was a difference between "No, it isn't." and "No, it is not"

I told him there was no difference in meaning, but possible one of the forms reflected a change in emphasis or nuance.  Although exactly what the difference in nuance was, I didn't know.

I googled the different forms, and I found some people speculating on various forums, but I couldn't find any one speaking with authority