I'll be honest. I liked Pearl best because the VS seeker allowed you to mess around with mon and experiment with new moves. Black doesn't have that, which severely hurt the replayability for me. Hell, hardly any trainers in Black can be fought again, and grinding in tall grass is boring.
I recommend B2. There are more Pokemon to catch, more trainers to rebattle (not many, but more) and a larger area to mess around in. The only downsides are a somewhat poor map layout (after a point they carve up the old Black map and only let you explore bits of it when the E4 are beaten) and that the daycare is only accessible post-league.
I still think B2 is a better choice, though, as it has more variety in it's Pokemon and more ways to mess around with them, like the competition dome in B2's Driftveil. It expands on Black is what i'm saying, and I had more fun in Black 2. Story in Pokemon was always secondary to the gameplay for me, but B2's story is about as good as Black's was if you're interested.
Plus, you can get a new (kinda) legendary Pokemon in-game in B2 that's not in Black, in addition to two of the legendaries from Black.