I stayed in the Hilton City Mainz for about 1,5 years. Therefore, I can recommend you the rooms at the top floor (501 for example) which have a little balcony. It is much more quiet there than in the Hilton Mainz, the rooms are in a better condition and you have only 5 minutes to the city center. The river view at the Hilton Mainz is nice, but you do not need that if you stay for a longer time.

Staff in the Hilton Mainz City is generally ok, but if you have any kind of problem, ask for Nadine at the reception (if she is still there). She is the one that is taking care most of the wishes of the guests.