I think the hammock might work for a short nap in hot weather. I have one in the yard. It lets air flow around you keeping you cool, and is a good place for an afternoon nap, but I don't think I could spend an entire night in one and wake rested. Especially in cool or cold weather.
I've used a pad of some type in the past and prefer that method. A tip I recently read about, but haven't tried yet, is to use one of the reflective shades that people use on the windshields of cars in hot weather to keep the inside of the car cool. They are quite light, cheap and take up little room in a pack. The reflective coating supposedly helps keep you warmer in cold weather. No promises that it works, but a thought.