Weight is my go to for any measurement I want to be reasonably precise, solids or liquids. my kitchen scale gets a lot of use.

that said, if this is a baked good like a cookie or brownie where you enjoy biting into the existing texture, I'd probably go with less precise and just cutting it by eye in whatever way seems less destructive. 1/5ths is a little tough though. I'd tend to go for 1/4 or 1/6 and feel better about the precision to cut something in half and then half again or in thirds and then half again.