I enjoyed both cities when I was there last month. I guess that was before all of these issues mentioned in the article. If you've already seen Frankfurt then Mainz could certainly make sense. There are plenty of things to see in Mainz. The Dom was closed when I was there due to some fairly extensive renovation work. Not sure when it is due to open again. But there are several other beautiful churches in the city. And I loved the Gutenburg Museum. I would say just find out what the real train schedule is and be conservative. It's about 25 or 30 minutes from Mainz to FRA on the S-bahn.