I choose WS foam. I like it because it seemed less difficult than cork. It may still have a LITTLE "squishyness" as some call it, but when ballasted with glue and all that holds it in place.
FOr larger areas like under switches and in yards, I bought the "craft foam 8.5x11 sheets" at the craft store in black to go there. It is all most exactly the same heigth and isn't noticeable when the trains shift from WS to the Craft foam.
Cork is supposed to turn brittle and dry out, the idea behind WS foam is that it doesn't. Only time will tell.