>>24815125BW has better music and player character sprites/art. You're also forced to use solely Gen V mons until post game, which I liked because it forced me to find new bros instead of just going back to the old standbys. It also felt more difficult than BW2 pre E4, but that might be because I was going in blind.
BW2 has more variety in what you can use while progressing, a better post game in the form of the Pokemon World Tournament, and was good enough that it is worth playing as a direct sequel to BW.
That said, if you were going to get just one, get BW2 as there is more to do post E4.
