Not a serious hanger, but have spent a few nights in my Byer mosquito hammock in a bushwack situation. I use a 20 degree rated bag. But I also have a fleece top and bottoms, that I keep in my bags compression sack. I'll put them in bag with me, and some times I'm glad I did. Nice to have a cozy outfit on for those late night bathroom calls, or to start the morning coffee water stove/fire.

I'd opt for the zero degree bag, unless your the type of person who runs "hot" when the sleep.

You can wear clothes to bed, if you find the 30 degree doesn't keep you warm, but adding clothes is easy in a tent, not so much when your hanging in your bear piñata ;D  Depending on tarp set-up and wind direction, wind will play a big role on whether your warm or cold.

I'd rather have to unzip the bag, than be cold all night. Most bags are generous in there warmth ratings. I'd error on warmer, all things considered ;)