That's the great thing about terrain building... you can use just about anything. There are plenty of great blogs on this forum from which you can gain some great ideas.
@Crowsfoot made an amazing watchtower once out of Coca-Cola cups! (and won the terrain building contest with it!).
I myself build a lot of my terrain pieces using wood. Partially because I have familiarity with it it (via Scroll Sawing) and partially because I am not nearly as skilled with insulation foam as many of the members on this forum.
It's a good idea to keep a box of potential terrain building materials. When something interesting catches your eye, put it a side for later.
Do you know the difference between these? Have you worked with both? Are they different in any way besides colour?