Both stop people seeing your e-label, but they’re very different:

• Unpublish — hides the e-label and puts it back into draft. The QR code still exists; it just won’t display the e-label until you publish it again. This is reversible — use it when you want to hide a label temporarily.
• Delete — permanently removes the e-label and its QR code. The QR code stops working when scanned, and this can’t be undone.

If in doubt, unpublish. Only delete labels you’re sure you’ll never need again. (You must unpublish a label before you can delete it.)