Overlays have a specific purpose; first of all, they can have multiple children while a border can only house 1. But most importantly, an overlay allows you to overlap widgets, position them in an arbitrary way, stack them one on top of another.
Border’s job is precisely what @ipninichuck described, no funny business. If an overlay gets the job done, that’s fine, too. Overlays may be a bit more performance hungry, this is probably negligible until you iterate through hundreds of them every frame, though.
@DamianToczek - if you want a proper border with an adaptive frame, look into 9slice - it’s described in the Styling Guide - Setting Image States section, explaining the use of DrawAs. Also, this ancient thread describes it in detail: https://forums.unrealengine.com/showthread.php?51664-UMG-9-slice-images-look-ugly-can-t-get-margins-right