Probably depends on the biodegradable fiber? Organic cotton uses a _ton_ of water. I'm not sure about viscose/rayon/tencel but I imagine it's often less water-intensive but it can be pretty chemical-intensive.

I don't know how bad nylon is to produce, but the durability it adds to clothes is probably an overall net win for the environment; compared to other plastics we consume, it does a lot of work (I've worn some items for years).

Note Prana has recycled nylon in their Brion II line, but unfortunately the quality is way worse; it pills immediately. Environmentally, I'd much rather buy non-recycled plastics that last a lot longer, so I consume/waste less overall.

But the tldr is that I don't know :)