I did not have a particularly bad experience at HR Mainz, though I wouldn't classify it as a particualrly good one either. Even though I handed the front desk staff my DIA card, they still got my name wrong and checked me in as someone else, which was apparent when I told them I was using a FFN for the stay. They would not (or could not) give me a river view room, though I had club access. Snacks and drinks, just so-so. The breakfast, same. I had stayed at the Hilton the night before on an award stay, and they gave me a river view room and full breakfast, as a Gold HH member.

If you do stay in Mainz, check to see if the train you want stops at Mainz Sud as this is the closest station to HR.

Frankfurt, and the Arabella Grand, probably would be the better choice, as the weekend rates are similar to HR Mainz's. Not sure about weekday rates, which can be quite high in Frankfurt.