Like all the paired games in the Pokemon series, Black & White versions each include Pokemon that cannot be obtained in the alternate version. Furthermore, Pokemon White contains an exclusive route, the White Forest, which contains a large batch of Pokemon. The equivalent in Pokemon Black is the Black City, which has more trainers but no wild Pokemon.
Note: Cottonee and Petilil are only exclusive in the wild to Black and White respectively. The unobtainable
one is available in each via an in-game trade.