It is always best to run programs off the internal (The Data in them can easily be stored on a fast external drive (It's the way most people use computers), however programs improve over time and get bigger, thus a larger internal drive is more beneficial. (1TB is the optimal for most uses with the current software that is available, and will allow for plenty of future software updates)
BTW: Most music software requires fast single thread performance rather than multi-core performance.

Bill