Bamboo and hemp are by far superior to organic cotton for quite a few reasons. Although organic cotton is natural, it uses far more water in both growth and production of the fiber. Organic cotton is a good alternative to standard cotton or the dreaded polyester, but it isn't the greenest way to go!
Bamboo and hemp are fast growing plants that don't require and excessive amount of water and don't pollute the environment in growth or production. The fabrics developed are superior to synthetics, and easy to care for and are incredibly soft.
What's wrong with synthetics? Many require the use of petroleum products, contain chemical residues that will be absorbed by your skin and pollute the environment during production.
You will find that most companies that create eco-friendly clothing also produce their clothing in a socially responsible manner, paying employees a fair wage and not employing children.