Well, short answer is to use SET all the time.
It gives a better quality if you want to re-zoom or modify after.
Here an exemple of what I did to illustrate what I mean.
Focus on the text "Reports" in the lower left area of the images. SET shows a clearly image.
I don'e even know why SCALE exist. I don't even know in what case you could use it or in what scenario would be better than SET.