While I have not been on either of the tours you are considering, I have been on the RS Berlin, Prague & Vienna tour in 2015. Fantastic by the way! And goes to both Berlin and Vienna. I loved this tour as it covers history, art and culture of this wonderful area.
I think you want to decide what you are looking to experience on a tour. I would love both of them and can understand your dilemma.
GAS - covers 3 diverse countries - gorgeous Swiss scenery
BoG - delves more deeply into German history and culture. I think it also covers the Martin Luther trail. This would appeal to you if you have an interest in the Reformation. Starts in Hamburg which friends tell me is wonderful, very modern and full of history, too. Ends in Berlin, which I found is an amazing city!
Also, you might buy Rick's guidebooks to learn more. Or, on the Watch, Read, Listen tab on the website, read some of Rick's travel articles on some of these places to see what piques your interest.