Used to express a preference or a correction, often contrasting with a previous statement. Common in both spoken and written Japanese.
Rather than coffee, I'd prefer to drink tea.
彼は怒っているというより、むしろ悲しんでいるようだ。
He seems sad rather than angry.
He's not so much a doctor as a researcher.
Rather than forcing yourself to go, you'd better just rest today.
むしろ stands alone to express a stronger, more decisive preference, whereas より is used within a comparative phrase like AよりB.
むしろ directly contrasts two options or judgments, while いっそ suggests picking a bolder, more drastic choice.