There was a facelift of the A4 in mid 1999 (commonly called the 99.5 update).
So it depends whether your car is a pre or post facelift as to which bumper (from the 98 or 01) is most suitable.
The visual difference is pre facelift bumpers have a three piece lower grill.
The post facelift bumper has one continuous lower grill with built-in foglights at each corner.
If you don't know what your original bumper looked like, another indication is the headlights, pre facelift have separated headlights and indicators and the headlights have a glass lense.
Post facelift have a 1 piece headlight and indicator assembly with a clear PVC lense.
The other indication is obviously the build date, but I'm not sure of the change over date.
If all you're concerned about is fitment, both bumpers should fit.
Both bonnets (hoods) will be the same - there weren't any major sheetmetal changes between pre/post facelift.