There is no difference at all in either usage or grammatical correctness. The two forms are interchangeable and both correct. It's the same difference between "There are no" and "there aren't any". Both can be used with equal correctness.

The difference is simply which contraction you choose to use. They are both the same grammatically and semantically because they are both contraction forms of "it is not." The first one takes out the "i" in "is" to for the contraction "it's." The second one takes out the "o" in "not" to form the contraction "isn't." They are both correct and each is used frequently.

yes i agree with them