Advantage is like around +4.5 bonus to hit, so u hit with advantage and enemy hit u with disadvantage, it is like +9 net, your comment is wrong.
Speaking about arithmetic mean (average) there is 3.3 difference. Standard roll has mean 10.5 while advantage or disadvantage has 13.8 resp. 7.2. Median for d20 roll is 10 or 11, whille median for advantage is 15 and disadvantage has 6.
