Rect Transform, adjusting size via width/height, or scale?

This feels like a simple question, but the documentation wasn’t too helpful.

A Rect Transform has both the properties, Width/Height, and Scale, X/Y.

Changing these values both resize the rect transform.

So, which one is the “correct” way of making the UI element the size you want?

Definitely width and height.
If you, for example, have a UI Image and with a sliced background, changing width and height will not distort the image. Changing the scale will distort the image.
Here’s how it looks with changed width:

Here’s how it looks with changed scale: