There are three main materials used to make rulers. Wood, metal and plastic. They all have advantages and disadvantages. Plastic rulers are perhaps the most useful as you can see through them, this lets you make sure that you have everything aligned much more easily. Plastic rulers are easily damaged by sharp knives, unlike metal rulers but, unlike metal rulers, they are cheap and easy to replace.
If you are going to be creating your own designs make sure that you have a ruler with clear measurement markings. Here again, the plastic ruler wins out. As the marking are on the bottom of the ruler, in contact with the paper being measured, there is less chance of parallax error. Go plastic. But buy three.
