Lot of differences. I had it described as "7-6 means you can get there, blow the whistle and point in the right direction, 6-5 means you can manage a game". Just remember what their titles are -
7 = Junior County Referee (junior as in experience or ability, not age!, maybe about 20%)
6 = County Referee, (so you'd expect most refs to be at this level as they would be the middle, majority 70%)
5 = Senior County Referee, (the County's most senior, experienced, top refs 10%
and not
7 = Passed the course (1%)
6 = Got promoted last year so just here cos I have to be for this year / didn't get enough games last year (10%)
5 = Standard Referee (89%)
Most I know / met / speak to don't get their 5 straight after their 6 especially if it's only your 3rd season or so (Qualified, first season, 7-6) - so go for it and good luck, but be prepared to not get it with the advice of "not quite / needs another year at 6" etc. Are you Senior over those around you - league / county?