One difference is the version exclusives.
Black 2: Magby, Magmar, Magmortar, Spoink, Grumpig, Registeel, Latios, Buneary, Lopunny, Stunky, Skuntank, Gible, Gabite, Garchomp, Gothita, Gothorita, Gothitelle, Vullaby, Mandibuzz, Zekrom, Black Kyurem
White 2: Elekid, Electabuzz, Electrivire, Skitty, Delcatty, numel, Camerupt, Regice, Latias, Glameow, Purugly, Solosis, Duosion, Reuniclus, Rufflet, Braviary, Reshiram, White Kyurem
In post-game, in Black 2, you have the opportunity to visit Black City and the Black Tower. However, in White 2, they are replaced by the White forest and the White Treehollow.
Sources: Experience and https://bulbapedia.bulbagarden.net/wiki/Game-exclusive_Pokémon#Black_2_and_White_2.
I hope I helped.