Which material would be preferred when constructing a semi-anechoic room when choosing between glass wool or polyurethane foam?

This question presents a great deal of difficulty, but because of the ambiguity in the question. There are two problems: 1) we don't know how "semi-anechoic" the room needs to be, and 2) there are a wide variety of polyurethane (PU) foams of varying "hardness" and, therefore, compressability. There really isn't a good answer here unless more specific condtions narrow the question. Sound is mechanical energy, and to be absorbed with a minimum of reflection, it needs to act against a "soft" material, that is, one with a low shear stress and high compressability. That way the mechanical energy can be transferred into and "used up" by the material. PU foam comes in such a wide range of "hardness" that no comparison between that and the glass wool can be made. And how compressed is the glass wool? The question leads to more questions without being closer to an answer. Take a quick look at the variety of PU materials by using the link to the Wikipedia article on the stuff.