Is one stronger than the other? More correct? Just curious, one of the many abstract things to pop into my head on the drive home today...
The biggest difference between the two is how many syllables it takes to say them:
This obviously matters a whole lot for rhythm and rhyme.
Sometimes (though rarely) it is worth avoiding the harsh 's' sound from it's not due to microphones' tendency to accentuate the
But in terms of meaning there is no significant difference. Using "it is not" would sound a tad more formal due to the lack of contractions and the overt effort required to say it that way.
They're very similar in meaning -- both are correct, both are contractions of "it is not", and both are fairly casual in tone. The differences are extremely slight, and easily negated in spoken speech by word emphasis.
Of course, contractions are often discouraged in formal, or technical writing.
Perhaps one might avoid "it's not" because it sounds a bit like "it snot". I can't think of any other reasons one might prefer one or the other.