I first want to add that I copied the following from an article that I read recently: "Ordinary fiberglass insulation placed between the joists will muffle sound to an extent, but the best approach is to couple this with drywall fastened to the bottom edges of the ceiling joists using something called resilient channel. These are sheet metal strips that allow the drywall to be anchored without it touching the underlying wooden ceiling framework. Itís also very important the tape all drywall joints carefully, since the tiniest cracks let a lot of sound through."
Personally, if it were me, I would install 1" to 1.5" styrofoam insulation against the floor and then, add bats beneath it. I believe an inch of styrofoam is equivilant to 4" to 6" of fiberglass insulation and it is easy to cut. One thing in its favor, styrofoam does not get down the neck of your clothing and itch.
I used the styrofoam on basement walls with great success.