I would say the main difference is scope: SymPy is aiming for traditional CAS territory: algebra with elementary functions, calculus, ODEs, elementary number th

I would say the main difference is scope: SymPy is aiming for traditional CAS territory: algebra with elementary functions, calculus, ODEs, elementary number theory, plotting. Sage aims to include tools for computations in all parts of mathematics, including all of the above but also more advanced things like group theory, representation theory, homological algebra, commutative algebra, homology of simplicial complexes, graph theory, Galois theory, algebraic number theory, etc.