The train from CGN airport to Cologne main station is 13 minutes. (at least three trains an hour)

From Cologne main station to Duesseldorf airport is about 40 minutes (2 trains an hour)

Cologne has more history, so there are more sights. The most famous thing is of course the Cologne cathedral, a must see, right at the main station, can't miss.

I don't know how much time your meeting on Wednesday takes but if it is not a big thing, I would try and do some sightseeing during Wednesday and then take a train to Duesseldorf in the late afternoon.
Get a hotel in DUS and walk the "Altstadt" (old town) for a bit, have some nicer dinner there.

If your flight leaves at 8:10am, I wouldn't want to be at DUS later than 7am and that is really early if you have to leave from Cologne.

The airport of DUS is north of the city and about 15 minutes by local train to get right into the terminal.

If you have any further questions, let me/us know.