Given I've got good 5th Gen knowledge I should probably give recommendation that you play Black and White first before playing Black 2 and White 2. The reason is playing the original pair before the sequels is that it will give you a better explanation of the Unova story and the two years between B/W and B2/W2.
Also, Black 2 and White 2 have a special feature called Memory Link, which is when you connect Black 2 or White 2 to a copy of the original Black or White. It unlocks special cutscenes that aren't present without it, the ability to catch the Pokemon that N used in the original Black and White, and it's basically a cool feature in general.
So yeah, the conclusion I have come to is play Black and White before Black 2 and White 2. You can experience the whole Unova story that way, and if you only get like say, Black and Black 2 (or White and White 2) you can get both Reshiram and Zekrom that way.