Eau de Guerlain (1974) is a chypre fragrance (bergamot-moss-patchouli accord) with a strong basil note. It has a good citrus note the top, but the basil makes it very green and a little spicy. The heart note is very floral: Rose, jasmine, and carnation.
Eau de Cologne Impériale (1853) is more of a slightly fougère scent (lavender-tonka bean accord, missing the typical oakmoss note). The top is very citrus dominant, with verbena (reminiscent of citronella) being the distinctive note. There is lavender as the heart note, and the base is cedarwood and tonka bean. The tonka bean can give some people a powdery impression, although the other notes do a lot to compensate for this. Eau de Cologne Impériale was made for the Emperor Napoleon III, hence the name "Impériale."
They are both very recognizable examples of the eau de cologne style, but the chypre/fougère difference is crucial. Also, the Eau de Guerlain is much more recognizably a modern scent in my opinion.
Hope this helps.