There are few differences between all 3 generations of the 7s. The second and third generations have lead free solder, different software assigned for each generation that can be swapped if you wish, and on the 3rd generation, changed wi-fi. Although there are a few differences, it still is a great idea and is recommended to have all 3 generations of the 7s in your AIBO pack or toy/robot collection as they are a marvelous piece of history, and also because other people like collecting the different generations of things that usually leads to the development, or final model.