While this would be best done in the Pokémon Gaming Central Quick Questions and Answers Thread, I think Black 2 would be the more fun pic, at least if you're one that wants an adventure that can go fluidly with plenty of Pokémon. Soul Silver is arguably more challenging, but it is terrible with training. If you weren't held up by the lack of post-game in White, Black 2's will likely make up for it with the PWT and Black Tower. And lots of other various post-game content. At a same time, a lot of the B2W2 haters are the big fans of the original BW, but please pay no mind to those that trash it for the story or being the same thing, it has lots to do - as far as the adventure goes it's more to BW than Emerald is to RS, i'd say.
But Soul Silver isn't too far behind, especially if you're a major fan of Johto, since it has walking Pokémon and a nice touch screen to take advantage of. And as I said, it does have another good helping of post-game, these two games rival each other for the best in the series on that note.
Anything else I could possibly insight you with?