It's got to be silicone.
Since the DivaCup has the most "It leaks!" reviews and I know that if she has a bad leaking experience she won't put out the cash for another one, I want to recommend the absolute lowest chance of leaking.
Lunette isn't yet in my comparison list--not relevant to this post.
Anyway, here's something I can't find: leak frequency generalizations and experiences and claims between Mooncup UK & Keeper Moon Cup.
Thanks so much!