It's a cost-benefit equation. For example, You could use a Quantum processor in a machine that has 10 DI an 8 DO, but for you this will be most expensive than if you use a Momentum processor, with the same end-results. These is the same that if you use a Ferrari engine in a Ford-T car.
In my opinion, you will use a momentum controller in smaller applications and a Quantum controller in bigger applications, according to the process needs.
1. total number of I/Os, for the moment and also for possible extensions in the future
2. needed RAM for logic
3. communication options for other systems (eg. SCADA,..)
4. price
Actually, quantum is generally used for large and complex systems. Momentum is more suitable for smaller systems and also it's generally used for distributed structures.