不是…就是【bú shì … jiù shì】

不是 A 就是 B says there are only two possibilities and one of them is true. It often describes a habit, and often with a sigh: every time you look, it's one or the other.

A guess between two

It also narrows a guess down to two answers: "it must be A or B".

不是…就是 or 要么…要么

不是…就是 describes how things are. A choice someone has to make takes 要么…要么.

• × 不是你去，就是我去，你选吧 → 要么你去，要么我去，你选吧

Not 而是

不是 A 而是 B rejects A outright. 不是 A 就是 B keeps both A and B in play.