For me, 20% less pay = 100% less saving at the end of the month.
Unless your income and outgoings have been fixed at their exact current levels for your whole life, there's nothing magical about their current values and they could change?
Don't get me wrong, both myself and my wife are both working 4 day weeks after the birth of our son (culturally, that certainly helps the discussion), and that extra flexibility let's you choose the option that's best for you, but there is an opportunity costs trade off between them all...