Personally, I think it makes more sense for White owners to purchase Black 2, and Black owners to purchase White 2, as they will be able to obtain the exclusives that were unavailable to them in the original versions, and gain access to the alternative of Black City/White Forest. Though if they were to get the respective sequels, they would be able to capture the opposite legendary (Zekrom in Black 2 and Reshiram in White 2).
And, uh... DarkGengar, you know Zekrom is exclusive to Black 2, right?
I'll be getting White 2 so I can get the exclusives I missed out on in Black. And fortunately, I own two other DS's, so I can obtain practically every Black 2 exclusive via transferring and trading. I can hold on to them in my Black version, and trade them to White 2 whenever I want to use them. Aaah, the benefits of an extra DS~ (Even if they're damn near exhaustion)