That depends. Are you concerned with plot and story development, or overall content?
B/W was completely superior in the former, where you had a long, branching plot that extended through the elite 4, while in B/W2, the entire plot develops and concurs after the 7th gym.
B/W2, however, has ultimately more content, and just proves to be more fun, extensive, and rewarding if you give it a lot of time. That in mind, I recommend B/W2.
I have claimed Alduin, the mighty dragon! He shall one day return to bring forth the end of time, and fulfill the prophecy of the world eater.