Both sit at the top of the Simucube 3 line on the same newest architecture, so this is really about torque and prestige. The SC3 Pro already tops our fidelity and build axes; the SC3 Ultimate adds professional 35 Nm torque and costs more than twice as much for it.
So unless you specifically need 35 Nm, the SC3 Pro is the base to buy - the same class-leading build and software, top fidelity, at less than half the outlay. The SC3 Ultimate is the halo, for professional rigs where the extra torque and prestige justify the cost.