Just as the question asks, what is the difference between Seventh Heaven and Cloud Nine? I know that both mean extreme pleasure or joy, but I'm wondering if they have different meanings when used in the same context.
The principal difference is that they belong to quite different era and etymology. 'Seventh heaven' is the most exalted level of heaven, esp. the highest and most holy or blessed of the hierarchical series of heavens described in Jewish and Islamic theology According to the Talmudic Hagigah 12b, the place where God dwells over the angels, the souls of the righteous, and the souls of those yet to be born. The OED has a reference from as early as the fourteenth century.
c1390 (▸a1325) Ipotis (Vernon) 82 in C. Horstmann Altengl. Legenden (1881) 2nd Ser. 342 Þe seuenþe heuene, seiþ þe storie, Is paradys aftur purgatorie.
Cloud nine is not even in the OED as a composite. My sense is that it is late-twentieth-century but I am unaware of its etymology. The Phrase Finder seems uncertain of its origin but tends to corroborate my idea of late-twentieth-century. (Unless the Buddhist idea holds water) See:
I think the meaning and use of the two terms differs in that 'seventh heaven' is usually used to mean that the subject is in a state of blissful happiness, whereas 'cloud nine' is perhaps used more to imply that the subject is in such a state of blissful euphoria that they are unaware of what's going on around them, and detached from everyday concerns.It tends to be applied to daydreamers - 'it's no use talking to him, he's on cloud nine'