Silicone is considered relatively environmentally friendly for a number of reasons. It is composed of silica (sand), oxygen, carbon and hydrogen. For medical (and other consumer) grade silicone, rather than using the old synthetic process which requires the evolution of HCl gas, they use a process which yields acetic acid (vinegar), a useful and nonharmful byproduct. Silicone itself does not degrade easily, is nonprous, and does not exude chemicals as other "plastics" do, however it is combustible.
Plastic is another story, depending on what plastic is used (it is not specified). If I knew what plastic is used I could give you a better explanation.