Ooh new wiki guides!! 
Looks super useful!
There is no difference when it comes to how it applies to MusicBrainz, imo they should be merged, see related forum discussion here.
tl;dr: 99.9% of the time “design” is credited, in a MB context, it is shorthand for the more specific terminology “graphic design”. There will be very rare cases where something like “industrial design” is credited, which is another discussion, but probably doesn’t qualify for it’s own relationship.