Fixed width for buttons or proportional with the text?

What is the best practice for a UI, to make all the similar buttons with a fixed width, no matter how long is the text (with the condition to fit) or make the width proportional with the text and add the same padding?

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Update:

Please check also this example on Material Design Guidelines that suggest a minimum width and flexible width.

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Dialog buttons Button height: 36dp Button top margin: 24dp Button padding: 8dp Button alignment: Right edge for LTR scripts Button width: Varies based on button text length

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It is this a general rule?

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