I'm getting both games (and the third game, when it comes out), as I've done for most generations. Whenever I don't buy both games to begin with, I end up wanting the additional game at a later point, and then I have to make a separate trip to go buy it. I figure I might as well save myself the time and stress and recognize that I'm going to want every main Pokemon game that comes out, so I might as well buy them while they're still readily available (I learned my lesson when I tried to buy Emerald a couple of years after release and couldn't find it anywhere).
Also, I would like to be able to trade locally to complete my Pokedex, and since I don't have any reliable Pokemon-playing friends (the only one who plays at all doesn't play much), I'd like to have both games so that I can trade with myself whenever I see fit. Wifi does help with the trading aspect, but there are times when it can be really useful to be able to securely trade without worrying about someone cutting off the connection.
Having multiple games also gives me a chace to get more event Pokemon, which is always useful, not that I have much time to go to events these days.
I'm still debating which game is going to be my main game. I'm leaning towards White at the moment, but I'm sure I'll switch back and forth a few more times before release. Either way, I'll probably try to build decent teams on both.