numpy/scipy have very many more users, but I don't think that they're really comparable: they're aimed at numerical computation for science, while SAGE aims to be a do-everything tool for mathematics. All of scipy/numpy is actually included in SAGE, and you can interoperate between numpy stuff, and, say, exact rational arithmetic, or group theory, etc.
And you can use R and Octave in a worksheet, too, but my understanding is that the point of SAGE is much closer to a superset of the types of functionalities that you find in SymPy. That is, that it is meant for symbolic manipulation.
numpy/scipy have very many more users, but I don't think that they're really comparable: they're aimed at numerical computation for science, while SAGE aims to be a do-everything tool for mathematics. All of scipy/numpy is actually included in SAGE, and you can interoperate between numpy stuff, and, say, exact rational arithmetic, or group theory, etc.