I think a little clarification is needed here. The Hyatt is closer to the Mainz Römisches Theater station, the Hilton Mainz City is coser to the Mainz Hauptbahnhof. You are much more likely to find a taxi at the Hauptbahnhof if you need it for luggage or difficulty walking. If you are taking a long distance train, it will arrive at/depart from the Hauptbahnhof, and not stop at Römisches Theater.

As to value, the Hyatt is close to $100 more than either Hilton, so the question is whether it is worth that much more. I think that depends very much on what you expect from your hotel.

My thoughts:

Hilton Mainz City (HMC) - extremely convenient to city center and Hauptbahnhof, decent rooms, views - nonexistent, decent breakfast, mediocre restaurant at other times. Nice staff.

Hilton Mainz (HM) - very convenient to city center, but not terribly convenient to either train station. Close to K-D boat docks. Decent rooms, some of which have a view of the Rhine (you pay extra), better breakfast than HMC, two restaurants, both of which are good. To my mind, the Brasserie at the HM is the best restaurant in any of the hotels, but it is expensive. If this is a motivating factor, you may want to check with them that it is still open, as I don't see it on their website. Nice staff.

Hyatt Regency - very convenient to city center and convenient to Römisches Theater station. Better rooms than HMC/HM some of which have a view of the Rhine (again, you pay extra), probably best breakfast of the bunch. The restaurant is the second-best, better than HMC and the Weinstube at the HM, but not as good as the Brasserie. Hyatt Regency has a spa, if you care for that sort of thing.

For me, I normally find that the Hilton Mainz has the best balance of cost to amenities I value, but I have stayed at all three more than once, and on any given visit, different priorities have changed my selection.

*Note: I have no significant status in either chain. Mainz is an occasional stopover point for me, not a destination.