The difference between two sentences is one of them "you are not a quitter" is like, I know you are not a quitter, is a fact to me that you are not a quitter but the other one is more assuming that "you are no quitter" you shouldnt quit but I dont know as a fact that if you are a quitter or not however I wish to think you are no quitter. I assume that this is how these two sentences differs in their contexts.
The first one is more colloquial. That's the only difference I see.
Note that this thread is from 2008.
cemm, welcome to the forum! I notice that you have mistakenly listed English as one of your native languages. You can change that in the settings.