The menstrual cup is an ecofriendly alternative to tampons and pads. Moreover, menstrual cups are made in medical grade silicon and are less invasive to the vaginal flora than tampons, whose chemicals care responsible for an increased risk of Toxic Shock Syndrome.
In the long run, owning a menstrual cup is also cheaper than other single-use solutions: its price is equivalent to a 100 tampons which is what an average person can use within a year.
You should pick the size of your cup depending on your size and the strength of your pelvic muscles, which is why the bigger model is usually recommended to people who have given birth.