If you're happy with SymPy, then Sage doesn't really give you anything. That is, it gives you SymPy, which you already had.

Sage is many things intelligently integrated, of which SymPy is just one example. Superficially Sage is a set of command-line and web user interfaces with a preprocessor on top of the Python programming language. The deeper part of Sage is a set of libraries that tie together many high-quality tried-and-true open-source packages by providing a unified API and object model (theoretically based in category theory, I think), plus a set of package-specific APIs for when those are more desirable. The packages include Maxima as the main tool for symbolic computation, SymPy as another, NetworkX for graph-theoretic operations, PARI/GP for number theory, and many more: http://www.sagemath.org/links-components.html