I've stayed at the Frankfurt Marriott in November and I thought it was a nice hotel. It is the closest Marriott property to the Old Opera and Sachsenhausen area. I believe it was around ~10 Euro each way by taxi. I've also only stayed one night so my evening was spending walking around the Old Opera / downtown area. I had dinner in Sachsenhausen area which is an area with bars and restaurants that have the German feel.

Again, I am not familiar with German but this was my experience.