The Dom, no doubt about this. It is the much nicer hotel, the by far better location (DIRECTLY in the heart of town and not in the station's backyard at a big bus station with plenty of people around you do not want to be with at night). The 4P is close to a better YMCA. Stationwise they are both the same, you take the train from Cologne Central Station to CGN, very easy, cheap and convenient.