As for poly vs. ripstop nylon, some people hate stretch and others like a bit. I'm 5'11", 165 lbs., and I really don't notice stretch. If I weighed more, it might be a concern. You may not know until you try both which you prefer.
Also if you get a double layer it will hold more weight and allow you to put off an UQ for a bit as you can use a pad or other insulation with less slipping. Also if you and GF will both be sitting in the hammock, I'd prefer the double layer for the extra strength.
Whichever you choose I am sure you will enjoy.