I agree with your analysis of how far each vehicle will get - in fact that's why I say the car on autopilot is more dangerous. A conventional car with cruise control enabled will 'fail early' and crash in fairly short order (most likely running off the road into a crash barrier or something) if the driver stops controlling it. A car on autopilot won't crash for some undefined but probably long time, which means that when things do go pear-shaped, it's much more likely to be on the road and in a significantly different environment from where it was driving.