Air Berlin only operates seasonally to Boston from Dusseldorf.
Lufthansa operates Boston from Munich and Frankfurt.

What is your routing? AB would be better if you wanted to get to DUS assuming the flight timings work as it is a direct flight. LH better for MUC/FRA as it is a direct flight. If you are connecting then the connection times may be very different.
What travel class?
Do you have status in any airline/alliance?

Hard to provide any useful advice without more specifics.