I have not used the bamboo batting myself (and never heard of the soy!) but I have felt examples done by others and loved the feel of it, they reported it was similar to working with a silk batt. Bamboo fabric is basically a form of rayon.
It is not an issue of mine, but some people concerned with chemical exposures are not very complimentary with the way bamboo is processed. I guess there are a couple different ways but we the consumer doesn't know which way we get.