Using the difference between a spur and pinion as a guide will be very confusing, and innacurate.
What matters is the gear ratio.
If your motor is running too hot, you normally need to lower the gear ratio. You can do this by fitting either a larger spur or a smaller pinion. Sometimes, an under-geared motor will run hot, so you can raise the gear ratio by fitting a smaller spur or a larger pinion.
In your example, 20/58 is a higher ratio than 19/61, and will probably make the motor run hotter.