Sarah for the wife...Peri for the mistress.
Off topic, and a bit ranty, and I realize it's not your point, but I've always hated that expression with a passion. Not only is it insanely misogynistic and chauvinistic, it purports that marital infidelity is acceptable because it's fun. As someone who saw his mother absolutely destroyed by his father's affairs, NO, it is not acceptable.
The above is not directed at you personally. But everyone, please just think about it.