Small Lunette Selene or Medium MeLuna Soft Ball Cyan... which is better?

Well I've narrowed it down to 2! Any suggestions or scenarios that might persuade me to get one versus the other?

I've never had a cup. Never even heard of one until I started researching reusable products to minimize waste and maximize the savings. Now that I found them I can't wait to try one! I'm 28, no children but married & active, I think I have a low cervix (or just not much room lengthwise). My flow can be like niagara falls on some days, but I don't know if it's enough to warrant buying a large. Comfort and staying put are my biggest concerns. Any thoughts on why one might be better than the other?

Also, is it safe to wash these with Johnson's Baby Wash?