Go for 250Mbit/sec with 1&1
I had a couple of tries with 1&1 Customer service and eventually with the English-speaking person, Mr. Kluge, who was very helpful and knowledgeable, I manage to change my current tariff from 100Mbit/sec to 250Mbit/sec in anticipation of Fiber optic technology.
I was so far with various Internet service providers in Germany, but in my experience only 1&1 has a steady quality and improvement of Internet services (DSL and Mobile phones), which is also cost-effective (kostengünstig).