Lets talk about living memory. Take the last 50 years...
From 1960 to 2005 Saints were the better side = 45 years
From 2005 to 2010 Pompey were the better side = 5 years
Go back further....
From 1885 when Saints were formed until 1920 Saints were the better side = 31 years (22 years really as Pompey were formed 13 years after Saints)(minus 4 years for WWI)
From 1920 to 1960 Pompey were the better side = 34 years (minus 6 years for WWII)
Grand total
Pompey on top for = 39 years
Saints on top for = 76 years (63 years, really as Pompey were formed 13 years after Saints)
Give it another 2 or 3 years and balance on the South coast will be restored.