The GOG store page lists D:OS as updated to 1.0.57 but the sticky post mentions 1.0.67 and even 1.0.68 so what's the real version available for DL? I would prefer to buy from GOG but I'm concerned that Steam seems to be the primary site for updates. Say this isn't so.

Currently both GOG and Steam are at 1.0.67, 68 isn't out yet. - Steam does update first, but GOG has been keeping up and is usually out the next day. I have not seen any game breaking bugs where you would need to wait for a patch before playing, so I don't think you should let the decide for you. I'd say buy it where you prefer and you should be fine.