Depends on your reason for wanting a CAS. What projects are you looking to use it on?
MatLab is primarily a (exhorbitantly expensive) matrix computation environment with the symbolic features graphted on. The engine MuPad is pretty good on its own but if you use it stand alone you still have the penalty of Matlab running in the background.
For some idea of the range of CAS tools available see here
"Maxima is a system for the manipulation of symbolic and numerical expressions, including differentiation, integration, Taylor series, Laplace transforms, ordinary differential equations, systems of linear equations, polynomials, and sets, lists, vectors, matrices, and tensors. Maxima yields high precision numeric results by using exact fractions, arbitrary precision integers, and variable precision floating point numbers. Maxima can plot functions and data in two and three dimensions."
Can Maxima really do all this? I guess I'll download it and find out for myself.