Material type normal vs previum, what's the difference?

I noticed when placing an order, you can select a material type. There are different color options, along with an option to select a premium material, which costs about twice as much. There is not much info displayed about what the difference is, other than the premium has a nicer finished look to it. Is this the only difference? Is there also a difference in the strength or flexibility of the material? Is one less likely to break or more likely to last longer?