Doing a B2B on the Allure or Oasis is a good choice. The ships are so large that you really can't appreciate them in just 7 days. I've never done a B2B but if it I were to do so, my first idea would be on Allure or Oasis. You can't get bored. Learn the ship the first week, and then relax, knowing the secret places on the 2nd week.