Not many mariners have a choice as to the number of watchstanders.
From the point of view of safety the most common hazard of 6 on / 6 off is sleepiness at the end of 00-06 watch.
The issues in this case evidently is sleep inertia or grogginess. Similar but the difference is that sleep inertia will diminish over time at the beginning for the watch but sleepiness at the end of the watch only gets worse.
It matters on the margin.