Justin, Are you sure your 2x4 trusses will support the existing roof plus your added materials? Are you leaving the trusses whole or dismantling them? Decoupling some way will yield more soundproofing. It will make little difference what absorption you use in your 2x4 cavity regarding sound proofing. (Plus, I'm sorry to say I don't believe you will get very good isolation.) Use the cheapest you can find, the lighter and fluffier is best for lower frequencies. Mass,decoupling first 2 principles. Hope this helps.
The Five Principles of Soundproofing / Sound Isolation