Two nights at Mainz and one at City. Both decent but nod to city.

Check-in: Mainz was fine, nothing special, allowed 10am check-in. City was exceptional - warmly greeted, explained benefits, room wasn't ready at noon so was told I could have a free beer in the lounge while I waited 15 minutes. Coupon for 2 drinks. Winner City

Room: Mainz - Upgraded to river facing room (even numbered rooms if you do online check-in) that was average and no temperature control except opening the window. City - upgraded to top floor (505) with nice room and sitting balcony. Very quiet. Winner City

Amenities: Both offered a fruit, some candy, and a rolled product. City gave a large bottle of sparkling water with the 2 regular bottles. City also gave a coupon for two drinks. Winner City

Breakfast: Only ate at Mainz which was good.

Both are decent hotels with City being more quaint and friendly with better room given an upgrade. I'd choose City unless the price was 10+ euro more.