I got hold of an Ocean and Earth sock cover for a 12'6 a couple of years ago (can't remember who I bought it from, some mob out of QLD). It's a bit of a PITA to stretch over a bulky SUP and the sock covers the handle so portage with the sock on isn't as easy as it could be. I only use it for long road trips and for that it seems to provide some protection against stone chips & stuff. A good zippered & padded cover will give you better board handling and protection, but on the other hand the sock doesn't take up much room when it's rolled up and put away.

Personally having done, known, sock and silver cover, i will always go the good covers its protected my boards a whole bunch more, on and off the cars, in and out of garage etc