Do you mean between the two models (1.8T & 2.8), or diffs across those model years?
If looking for diffs across model years, there were numerous interior changes. Many changes took place between '98 and '98.5, all the way down to little things like the redline indicator on the tach (much brighter and more legible at night in '98.5+) and the contour of the climate control buttons (concave pre-'98.5). '98.5 also was the first model offered with the "Atmospheres" interior choices (on the A4). The '98 vs. '98 FAQ offers a good guideline as to the differences between those models, interior and exterior. For '99, the cupholders were relocated and I believe tip controls were available on the steering wheel. The head rests got a small adjustment button on the side (I think that was early '99) and on the outside the passenger rear-view mirror became "full size". Other than that, not much else changed. '99.5 Again saw major changes in preparation for the "all new" 2000 model, which I'll leave for someone else to detail. : )<p><a href="http://registry.audiworld.com/audi/registry/details.asp?car=909"><img SRC="http://www.planet42.net/images/sig_pic.gif" ALT="Rob O" BORDER=0></a>