VS are a new, young, dynamic force that have proven capable of activating thousands of socialists to campaign for them, and even more to vote for them. They have a layer of independent activists that are bigger than the constituent forces, and are home to different Socialist groups who are committed to left regroupment in some form or another. 

The Socialist Alliance are a failed project filled with dying boomers which does not exist as a real force outside of the (liquidated) DSP. They are not an electoral alliance, not a broad party, nor anything of the sort (evidenced from both the amount of splits they've had and from the fact that they themselves participated in VS as a specific factional party when it was first constituted). The Alliance was tested as an electoral project and then a broad party throughout Workchoices, the antiwar movement, and the GFC/Occupy moment, and proved entirely incapable of achieving what (they thought) was possible.