Extensive marketing shows that specialty insulation products isolate sound much better than standard fiberglass batts. The problem is extensive sound testing shows that specialty insulation isolates sound just as well as standard fiberglass batts. The use of insulation in a sound rated assembly is crucial, however, the insulation type is not. As long as the insulation is light, fluffy, and sized to fit the cavity, then it will provide the proper amount of absorption.
Specialty insulation manufacturers will dispute this of course and often provide an STC rating with the specialty insulation in an assembly. What is missing though is an STC rating comparing the specialty insulation to fiberglass in the same assembly. The absorption rate will also be touted for superior performance to fiberglass, but labs conduct absorption tests with the insulation exposed. Once rigid mass covers the insulation (as it would in a wall or ceiling), the differences in absorption coefficients are equalized.
Specialty insulation does have a place in construction for various reasons, increased energy efficiency, fire rating, easier installation, and fewer chemicals depending on the insulation. However, for sound isolation, save money and buy the cheap stuff.