What difference?

[civilseal]

Whats the performance difference between a 5v 1.5A 1.25Nm stepper motor
and a 3.1v 2.1A 1.25Nm motor?

Both controlled by a chopper driver at 24v

[shadrag]

Without detailed specifications for both motors it's impossible to tell exactly.

Assuming you are driving them at their rated current and 24v, I would think the second one would perform better.

[jeffs555]

Generally, higher voltage lower current drives have more inductance which causes the torque to drop more rapidly with speed. Comparing them in the same application with the same supply voltage you would expect the lower voltage motor to work at higher speed. However, like shadrag said, you would have to look at the torque curves to tell for sure.