Linen is also a lot easier to work with. Hemp wovens are not very stable at all; the fibers slide around on each other, so you can easily end up with skewed pattern pieces. It also ravels easily. The 14oz hemp twill I’ve got is quite nice, but it’s a challenging material to work with if you’re used to 10-12 cotton denim.
I believe–IIRC, and it’s been a while–that Tencel is either a specific process for making rayon, or is a brand. What you should be looking for in rayon that makes it better is closed-loop manufacturing. But US product labeling doesn’t require that kind of information; you’d have to ask the company that made the apparel, and they’d have to ask their mill or reseller.
Finding information about how green any part of the sewn-products industry is is very nearly an exercise in futility, unfortunately; companies don’t have to have the information, and they don’t have to disclose it.
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.
Oh, and I think that more linen is being grown now, since flax seed and flax oil seems to be more popular as general food items now. I use a lot of flax meal when I make my grain-flour-free/sugar-free banana bread, because it helps prevent the ‘bread’ from feeling excessively dry. So maybe we’ll start seeing more linen in the market as a byproduct of the flax seed and oil production?