I had a seven night stay at the Four Points Dusseldorf Central in February.
It is in the style of a basic European downtown hotel. Small rooms that were very comfortable. Friendly staff, free breakfast daily. No restaurant however, there is a bar in the lobby.
The hotel is just south of the KO and a few blocks from the train station. There are decent friendly restaurants nearby.
With just a one block walk you can catch a street car and be in Alstatd in about three minutes. I walked there once but after working at a trade show all day it was a bit far.
If you are attending a show at the Messe, the streetcars are free with your show tickets. This is much easier than a cab.
I have not stayed at the Sheraton in the airport. I prefer to be in town and try to experience life as a local.
Have fun and drink the Alt beer.