I think spectacular is mainly used as an adjective.
I cannot think of an example where it is used as a noun in modern day to day English.
It was a spectacular show!
The show was an amazing spectacle!
So I don't think (in noun form) there is much difference, but you won't see spectacular as a noun very often (or to be more exact, I don't see it as a noun very often, and therefore I wouldn't expect you to).