Hi kahhong,
It is advisable to give some context when you ask a question.
Your N2 is wrong, because you've got no noun; then, what is the article for? When you compare two things you ask:
What is the difference? I wouldn't say N1 is wrong, but an example would be a bit far-fetched:
You compare several pairs of things - A and B, C and D, E and F. Hence, we have 3 differences. Anyway it sounds odd to me.
That's how I see it.
Cheers.