Cotton hammocks are widely considered to be much more comfortable than hammocks made of synthetic materials such as polyester. The reason is that the fibers in cotton are softer, and also because cotton is more breathable than synthetic fibers, meaning more air can circulate and keep you cool. Cotton is also a bit more pliable than synthetic materials, meaning the hammock can stretch more, and conform to your body. The more your hammock stretches, the better it will hold your body without becoming uncomfortable against your skin.
One major problem with cotton hammocks is their ability to absorb water and hold it. This makes them prone not only to being wet when you want to lie on them, but also more prone to damage from mold and mildew. Synthetic hammocks tend to dry faster, and are not easily damaged by water.
Cotton can also be damaged by the UV light from the sun, making it a choice for indoor hammocks, but not as much for outdoor hammocks. Since cotton can easily be damaged by the sun, it is recommended that cotton hammocks not be left outdoors. So if you do use a cotton hammock outdoors, you should bring it in when you’ve finished using it. If you will be using your hammock only indoors, then this won’t really be a concern for you anyway, unless the hammock will be near a very sunny window or in a sun room.
One final issue with cotton hammocks is that they are a bit less durable than synthetic hammocks, not just because of their susceptibility to water and sun damage, but also because their strings are typically thinner and may break easier than the strings in a synthetic hammock.
If the utmost in comfort is your primary concern, and you don’t mind bringing your hammock indoors when it is not in use, then a cotton hammock will probably be what you want. If you need something that will be used primarily outdoors, and will require less care and maintenance, then you should probably opt for a hammock made of synthetic material.