If you are planning to meet business associates for dinner in Frankfurt, it's best to get a hotel in the central area of the city and to book your restaurants in advance. If you prefer to relax in the evening and have a quiet, spontaneous dinner alone or with colleagues who are staying at the same hotel, the Offenbach location is sufficiently removed from the fair so that you can walk into restaurants and get a table without a reservation. My experience is based on 10 years of attending the Frankfurt Book Fair (which I believe is one of the more heavily attended Messes) and a few times at the music fair as well. I have stayed in many different hotels, including the Marriott across from the fairgrounds, but the Arabella Offenbach stands out as a favorite--although it's a bit pricey compared to some other choices in my budget, which tends toward the middle and lower priced options. It's also reasonably convenient for train or taxi access to dinners in the city.
The convenience of a hotel with transit access to the middle of the fairgrounds is not to be underestimated, particularly since my company tends to exhibit in Hall 8, right next to the train stop, but a LONG walk from the city entrance to the fairgrounds.