Fra is nothing really to see about. )

Defenetly vote for Cologne, you will love the church and the surrounding, also Berlin as well as Munich are just some hours away by train.

Munich is a bit more touristy if you are into "wow - how old of a building" and a lot of museums which berlin does have as well. it is more of a personal choice.

I would take a look at the offerings on the website and try to decide which kind of additional programm you want at the site.

Berlin should be less expensive than Munich or Fra.