DEFINITELY go with Dom Hotel, I was prebooked into corner room (called "suite") as SPG Gold. Great room, fantastic views. Right next to Dom, very close to a bridge over the river, and short walking distance to train station. ^
The hotel is on the opposite side of the Dom's plaza from the train station. So if you have light luggage, you just go up the steps and across. If you are rolling the luggage, you have to go around the steps to the front, where it is sloped, and then cut across.
It has the best views of the Dom and is the 2nd closest hotel to the train station after the Excelsior. Close walking distance to riverside and shopping. Not sure in terms of business areas.
Take the train to the airport. And if you happen to be flying out CGN>FRA>XXX on LH you can even check in and check your luggage at the train station. The luggage goes straight to the plane in FRA.
Enjoy
