I find the double layer hammock to be much more supportive on the back. I get much less shoulder squeeze and just an altogether more comfortable lay. They also have more resistance to insects and allow a pad to be placed between the layers where it won't shift around as much.
I think something should be said for the different weaves of nylon available. I not an expert but from experience it seems that oxford, taffeta, & ripstop all seem to stretch different even when made from the same denier nylon. Stretch seems to be the enemy of comfort in a hammock.