It's not that simple. The fabric most commonly made from bamboo is rayon, and rayon can be made from any cellulose fiber. Most rayon processes are actually pretty awful; they produce a lot of waste that's not great for the environment. Rayon--regardless of the source of the cellulose--is weaker than cotton, and tends to tear very easily when wet. You can process bamboo in a way that is much more environmentally responsible, but then you get a fiber that's more like linen rather than cotton. But very little bamboo fabric is made that way.
Overall, hemp is probably the most environmentally friendly fiber out there. It's not perfect, but it requires less pesticides, can be used as part of crop rotation (for the few farmers that do rotate crops), and needs less water to grow. It also grows in more climate zones. The fibers are harder to work with, and water is usually required to process them to a useful state, but you get very long staple fibers that are quite strong.
It's not that simple. The fabric most commonly made from bamboo is rayon, and rayon can be made from any cellulose fiber. Most rayon processes are actually pretty awful; they produce a lot of waste that's not great for the environment. Rayon--regardless of the source of the cellulose--is weaker than cotton, and tends to tear very easily when wet. You can process bamboo in a way that is much more environmentally responsible, but then you get a fiber that's more like linen rather than cotton. But very little bamboo fabric is made that way.
Overall, hemp is probably the most environmentally friendly fiber out there. It's not perfect, but it requires less pesticides, can be used as part of crop rotation (for the few farmers that do rotate crops), and needs less water to grow. It also grows in more climate zones. The fibers are harder to work with, and water is usually required to process them to a useful state, but you get very long staple fibers that are quite strong.