denser the better, but even the densest polystyrene will break off rather
easily, so i suggest you do what they did in the book. they took black duct
tape and taped the edges to strengthen it against damage.

or you could glue balsa wood to the edges instead

polystyrene would be a much easier medium to work with then paper mache. i could see it being really difficult to do the rivers with paper mache.

for the tiles with the rivers, you will want to strengthen the bottom of the polystyrene with a board. otherwise the tile will no doubt break right along the river at some point.

good luck and when you get around to building it give us some pictures in a wip tread!