I would not go this far. KOELN Marriott is a nice hotel; rooms have a contemporary design and are overall nicer than rooms at DUS Ren. My impression is that KOELN Marriott has quite a fluctuation of trainees at FD which can lead to CS issues. No such experience at DUS Ren.
Taking the great rates into account one can get at both hotels you most likely will be ok at the one or the other...