ArcIMS Image Services allow the user to buffer and select features from the same layer.
If you buffer and select features of the same layer with an ArcMap Service you will get erroneous results or the request may cause the service to timeout.
ArcIMS Image Services create dynamic layers to draw the buffered features, the buffers, and features selected by the buffers.
ArcMap Services do not create dynamic layers. This inability may cause the results to be incorrect.