iop95, .01 ohms and 100A gives a voltage drop of 1V, in the old days moving coil meters were calibrated using 100mV, i.e. .001 ohms at 100A giving dissipation as 10W. If you are indulging in electronics, then using a opamp, the shunt can be as low as .1 milliohm (10mV), gets to be a problem to find a resistor low enough in value. One advantage of the other systems is that the current is floating, i.e.not tied to an output line.
You don't say what the main welding supply voltage is, but what ever it is its going to drop by a significant amount at a current of 100A. So it could be an idea to monitor this and use this as the "current" sensing.
Frank