Find your niche.
It depends. But right now, Black 2. I once thought that White Forest was a good point to get White over Black, but then I thought maybe this time it would be refreshing to see the other side, the other universe, instead for a change. If certain Pokémon become version exclusives and I like one more than the other, then I'll get whichever it is. But then I can't really decide if I want to get the sequel to my main game or simply another form of Zekrom for old times' sake (since I reckon they'll require you to catch the Kyurem form again). Black Kyurem's technically a version exclusive in itself, and again version exclusives is one of the main reasons I base on choosing a version over the other.
Maybe I ought to get both if I save up enough.
Or if my brother gets one then I'll just secretly play that after I get the other.