Now this is one of those hard decisions you have to make, but personally I prefer playing the original Black and White first before you try Black 2 and White 2 so that way you can get a better idea of the Unova story since B2/W2 takes place two years after B/W.
There's also a neat feature you can use called Memory Link, which if you link it up to the original Black or White, you can unlock cutscenes in Black 2 or White 2 that otherwise wouldn't be seen without it. Also, you can get all the Musical Props you earned in the Black or White game linked at once by talking to the man in the Musical Theater in Nimbasa City. Memory Link also unlocks N's Pokemon which all come with 30 IVs in every stat (neat!) and all make good additions to your team. They have a green sparkle when they enter battle, sorta like a Shiny Pokemon. And after beating the game, you can battle Cheren and Bianca with their original Black or White team based on which starter you chose in that game.
I'd recommend playing Black/White first, then do Black 2/White 2.