There is quite a difference in the way that expanded polystyrene (eps) and extruded polystyrene (xeps) are manufactured.
http://www.dyplastproducts.com/newsl...ore_xpseps.htm
I've had mixed results in casting aluminum with both types. Can't conclusively blame the foam, though. Seems like everything has to be 'just right' when casting. Temperature, pour rate, proper venting, etc.
The extruded polystyrene does have a much smaller cell structure and will leave a better finish ( if everything else goes right) .