Covers in cover view are displayed in a fixed size box as described in the FAQ answer
What is the difference between "portrait" and "square" in cover view?. The "stretch to fit" setting controls whether the cover will completely fill the box or whether its "aspect ratio" (its original proportions) will be respected.
If you choose "stretch to fit" the cover will be stretched and distorted enough to completely fill its container. Sometimes this makes the covers look odd, especially if you are using square boxes. If you choose "preserve aspect ratio" then the cover will be displayed so that its largest dimension matches the box. The smaller dimension will not be grown, so the image will not fill the box. This option can make the covers vary in height or width.
Which display someone likes is highly personal and can depend on the covers in that person's library. Try them all.