Hi rob,
Kev here, Sarahs (OH) they are the same product and both will melt with extreme heat (flame) and they will also melt when subject to solvents such as super glue, petrol and acetone.
If you are using it to build backgrounds or hides the layers of grout will provide insulation from the heat of a basking bulb and the PVA will protect it from any cleaning solvents or humidity.
Check out a previous post Denniss new viv to see the results that can be achieved with the grout/pva/styrofoam mix
The main thing to consider is the adhesive, if it can be painted over with household emulsion paint it should be ok to use with styrofoam
Hope this helps.
Regards Kev.