I've used both of these foams before and can work well for different things, but I just wanted to let you know they the foams he talked about are all polystyrene, if you're trying to avoid it. Lots of people have used it with no apparent ill effects but I decided to stop using it. It's up to you.The type of "rocks" or structure that you are trying to build plays a role in the type of foam that is best suited to your purpose.
White styrofoam is best suited to making craggy, "sharp" featured "rocks" and are best created by melting the foam into the desired shaped w/ something like a soldering iron. White foam doesn't carve nearly as well w/ a knife blade as the blue or pink stuff.
The denser blue and / or pink foams are best suited to making rounded, eroded looking structures that are made with a carving knife and sandpaper. A soldering iron does not work as well with these foams either, it leaves strings (like pizza cheese) when chut w/ a hot knife.