These sit in different classes, so the fair framing is need versus want. The Alpha EVO 12 is a 12 Nm mid-tier base with a zero-cogging 5-pole servo that reviewers rate as the smoothest signal in its class - a lot of base for $519. The SC3 Pro is the flagship, topping our fidelity and build axes with more torque and Tuner 3.0, and it costs more than twice as much.
For most drivers, the Alpha EVO 12 covers the actual requirement: strong, clean 12 Nm force feedback, active cooling for long stints, and a signal that gives up little to bases far above it. Spending up to the SC3 Pro buys real gains - more torque headroom and the last increment of fidelity and build the mid tier cannot match - but they are gains you feel at the margins, not a transformation of the everyday experience.
So the split is about ambition and budget. If you want a flagship and the price is not the deciding factor, the SC3 Pro is a magnificent base and the reference for feel and build. If you want most of the experience for a fraction of the outlay, the Alpha EVO 12 is the value-smart pick, and the £600 you keep buys a lot of wheel and pedals. Both are good; they are just aimed at very different buyers.