Well, John, you should have known better than to use "voltage" and "amperage" in the same post on this board. This place is so overloaded with electrical engineers you can't swing a cat without hitting one. Anyway, while those other guys argue about how many electrons can dance on the head of a pin, I'll throw in my own two cents.
First, can we agree that when it comes to starters, faster is always better? If so, then current is really immaterial. The faster starter wins every time.
Second, less current isn't necessarily bad. Less current means less voltage drop between the battery and the starter and that means more volts to the starter.
That said, I can show you that under every condition, the twelve volt battery will deliver MORE current to starter than the six volt battery. Let's consider three data points: The starter at zero rpm, at the steady state cranking speed at six volts, and at the 12 volt steady state.
Zero rpm: When you first hit the starter button, the starter is essentially a dead short. The current is only limited by the internal resistance of the battery and starter, plus any resistance in the cables, relay and connections. So even if the internal resistance of the 12V battery is double that of the 6V battery, it's still going to deliver more current to the starter than the six volt battery. Ohm's law.
Steady-state six volt cranking speed: When the starter has wound up to full speed, it's reached an equilibrium where the starter torque equals the torque required to turn the engine. It can't turn any faster because the starter's back EMF equals the battery's output voltage minus the voltage drop through the battery, cables and starter. HOWEVER, if we could instantly crank the battery's voltage from six volts up to twelve volts, the current through the starter would increase, because now we have more battery voltage than we have back EMF and voltage drop. That's because back EMF is strictly a function of motor speed. The increased current from the twelve volt battery results in increased torque, which in turn causes the starter to accelerate.
That leaves us with the third data point: The steady-state speed at 12 volts. It turns out that the six volt battery can never get up to this speed because of back EMF. So it really doesn't matter what the current is at 12 volts because you can't get there with six volts. Back to my first point: Faster is Better.
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