For this example, let’s assume you have picked a small NEMA 23 stepper motor which outputs 0.65Nm torque and requires an input current of 1.5A at 12v. Both the ZDSP and ZD4 controllers can handle this power level but they have different features which may have an impact on your choice. The ZDSP offers microstepping up to 1/16th microsteps whereas the ZD4 can go up to 1/128th microstepping. The ZDSP features a 24 bit speed resolution whereas the ZD4 has a 20 bit speed resolution.
In practice the key to making the right decision is to prioritise the performance features which are most important to you. If you needed exceptional positional accuracy and smoothness from your motor then the ZD4 Stepper Motor Driver would be the most useful. However, if finding and maintaining an accurate speed was the most important factor then you would be better with the ZDSP Stepper Motor Controller.