Both. I prefered 2K football when it was out, so I'm happy they're back.
But I believe competition will bring out the best from both companies. I'll by the one I think I'll like the most. All things being equal, I'd go with Madden because of the familiarity with NFL players. But it's unlikely all things will be equal. If the differences are similar to 2004/5 offerings, I'll be buying 2K's game.