I wholeheartedly agree with the recommendations above about the Welcome Hotel. It's been a few years since I lived there, so it's possible conditions have changed, but the hotel is relatively new and very well-located -- less than a 5 minute walk to the Luisenplatz. I don't really like the stretch of the Rheinstraße between the Hbf and the center at night, so for that reason alone I would pass on the Maritim.

(Completely off-topic, but my favorite Döner shop is in Darmstadt. Getting hungry just thinking about it...)