I highly recommend looking at the comparison of the two mentioned in response to this. It's a bit out of date, though. It has a reference to SymPy 0.7.2 upcomin

I highly recommend looking at the comparison of the two mentioned in response to this. It's a bit out of date, though. It has a reference to SymPy 0.7.2 upcoming, but it's already past that. Basically, Sage is more comprehensive as it pulls in a lot of different packages. SymPy is designed to be a Python CAS that you can use with other Python projects. Also, Sage doesn't directly support Windows, it requires the use of a VM image instead.