On the upside Roxul won't mold but it does not do much for sealing up the concrete. It is when you get a cold foundation and warm interior that meet up and cause condensation on the concrete. I would worry about musty smelling conditions.
There is a thread here about finishing basements. Yes I know, another forum to go through.
There are a couple types of foam that could be used. XPS and EPS. One is extruded poly (pink or blue stuff) and one is expanded poly (like styrofoam cups, etc). You may get away with EPS as it is cheaper. But you will need to look at R value, permeability, etc to really determine if it is right for this application. If you want the above article on basement finishing PM me with an email address. I will send it to you.
Also you will need to provide a fire barrier (drywall or concrete board) over the foam of choice. Factor that cost into the overall cost as well.
I think the bottomline is that using FG or roxul may work but if mold/mustiness develops later, you will have to redo it unless you are doing a quick renovation to sell soon.