False and forced choice
The length of the gathered end hammock is only one factor in the comfort. There are plenty of comfortable 9' hammocks. Even shorter ones.
Don't forget that the longer hammock will require a longer, heavier, and more expensive tarp, too.
If there were some breakthrough in design or engineering that made longer hammocks possible only now I would think that longer is likely better. But, that hasn't happened. If longer were necessarily more comfortable, they would long have been available.