If you need programmability or interoperability, Sympy is way nicer. If you just want an interactive symbolic calculator, Maxima is fine but sometimes quirky (has odd conventions due mainly to its age). As heisig points out, Maxima can be quite a bit faster (but I run into slow things with it too). Using Maxima via Sage is in some ways the best of both worlds.

You may also be interested in SymEngine: https://github.com/symengine/symengine