Homosote likely would be more sound deadening over plywood. Some people cut the homasote into being roadbed, bevel it and then ballast the track, others will cover the entire table with homasote (which allows you to do things like, for example, cut out the homasote to 'sink' buildings down below the surface layer). On my layout growing up I had ceiling tile on the table, worked pretty well and is usually a lot more available than homasote. Sometimes people will use a sandwich of cork/rubberized roadbed over homasote, which further will reduce sound.
The key thing is to try and isolate the track from the plywood, don't screw the track through whatever substrate you use into the table. My opinion, for what it is worth, is to glue whatever you are using to the plywood, then ballast the track using a binding agent (usually white glue in water with a tiny bit of alcohol), which will secure the track to the roadbed. This should help isolate the track from the plywood, and help deaden the sound.
With the layout I am attempting to build, I am using 4" of foam over plywood to allow me to more easily do below track features, and I'll likely use some form of roadbed, too, I have heard that foam isn't particularly good at sound deadening.
