You can either go for Black or White. The Story remains the same, except that we catch Reshiram in Black and Zekrom in White. And we have a Battle tower related city named Black City in Black, and White Forest in White where you can find Pokemons from earlier generations.
I advice you to take stick one game, and stick to the same game's counterpart. For instance: If you chose Black(I chose Black), continue with Black 2 , you can get Zekrom, which was not available in Black. Meaning if you chose same game and it's part2, you can get both Legendary Dragons. Kyurem is available in both though.
Only take both Black and White(and similarly Black2 and White2, if you want to get version exclusive Pokemons)
Black/White starts with Ghetsis, and 'misled' by N, where they sought to free Pokemons from people.
In Black2/White2, Ghetsis Team Plasma has returned. I don't won't to give you any spoilers. So I'll leave you with this
In Black2/White2, The Kami Trio: Landorous, Thundorous and Tornadous can change forms. And Kyurem can fuse with either Zekrom or Reshiram, according to the game you chose. Black2/White2 also has hidden Grottos, which allow to get some items, or Pokemons with different ability(which are different from other 4 gens)
Like I said, if you're taking Black, buy Black2, not White 2. Buy Black2/White2. Of course, it has better features.
And uhh, did I told you, in Black2/White2 we can attain Shiny Pokemons, part of main game. Shiny Haxorus at level 60, and baby Shiny Gible/Dratini, according to the game you chose. No cheats, no hacks, no finding in the grass for hours :D