For anal toys I will always recommend silicone. It truly is the best for back door play. Most of the other materials will hold onto smell and/or you will need to use a condom. Plus it will last longer which means more play time.
Silicone, glass or steel would be my recommendations for your butt. I agree with Dwtim about the safety aspect of the material. That alone is the reason for my vote. Good luck in your decision making.