which stepper motor driver is best for robotic arm?
This is just bad documentation on the part of the board manufacturer.
The setting you point to is to do with the rate of change of switch current and NOT the torque setting when stationary. 

My recommendation is:


Read the datasheet on Decay Mode and Torque settings. 
Use only switches SW1, SW2, SW3 to set the winding current. This is non intuitive, but what they did was include Decay in the current settings and simply confuse the settings issues. 
Set S1=S2=Off ...this is Normal 0% rate change speed (these are DCY1, 2)
Set S5=S6=Off ...this is Normal 0% fallback Torque setting (these are TQ1, 2)


Just to make sure....here's an image of the ones I have:



I did some measurements on a board using a 3A 2.8 Ohm Long Stepper motor with a 24 Volt power supply. 
I hooked the inputs to an Arduino Nano running Kiss-Stepper as a test utility.
All DIP switches (1-6) to OFF
I measured the current on Full step just after turnon. 

Results were as follows:
Supply Current 220 mA    Winding(V) 1.6   Target Current 2 A  Actual 1.65 A
Supply Current 370 mA    Winding(V) 2.2   Target Current 3 A  Actual 2 A

Hope this helps.