Peter,
If you have an opportunity to get a 90mm 2.0 APO Summicron R lens, I think you'd be happy with it. I use the 100mm 2.8 Elmarit APO as well as the 90mm 2.0 APO Summicron lens. Since the 90 is much smaller, provides a little brighter viewfinder image, I find it's a nice lens to use out in the field. Don't get me wrong, the 100mm 2.8 APO Elmarit lens is a superb lens as well, and picture wise, it's not easy to descern much difference optically speaking between these two lenses. The bokeh of both of these lenses at F/2.8 is similar, but with the 90mm lens,it has better correction of distortion at the outer edges. The focusing with the 90mm is a little easier since less effort is required to rotate the focus dial. Moreover, the 90mm is about half the length of the 100mm lens.
For macro shots, the 100mm lens is superb and is probably without peer amongst other macro lenses in the 35mm format.
Dale