If Error Sans and Fatal Error Sans were to fight, what would be the most likely outcome and why?

The most likely outcome is a swift and decisive victory for Fatal Error Sans. Error Sans's attacks, while disruptive, would likely be ineffective against Fatal Error Sans's reality-warping abilities and code manipulation. Fatal Error Sans could simply erase Error Sans from existence or rewrite his code to neutralize him. The power gap is simply too vast for Error Sans to overcome.