The SC2 Sport nearly doubles the torque (17 Nm versus 9) and brings Simucube's class-leading build and True Drive software, at twice the price of the Sport 9. On the flagship axes it leads clearly; the Sport 9 answers on value and as the cheapest entry to Simagic's encoder.
So it is budget and ambition. If you can afford the SC2 Sport, it is a base you will not out-drive for a long time. If $399 is the number, the Sport 9 is a good start with the Simagic ladder open above it.