There auctually was an AU 'update' which came out 30th March 99 which you can read about Here.
The AU2's got bigger and better brakes than the AU1, CD player became standard across the range. There are a few other things as well which I can't think of off the top of my head some are styling changes like the high series bonnet was used across the range on AU2(except XR) and clear rear taillights and others mechanical.
I just bought my AU2 for a couple of reasons, 1 being price, good value for money, the other because in my eyes the AU2 looks alot better than the AU1.
It depends on which model your looking at getting, weather it be forte, XR, Fairmont,fairlane? or something else.