Lekky is right, football all the way, but not American football. Football is popular because it takes a total athlete to be good at the game, its more than just sheer physical performance, it requires skill and incredible intelligence to be a great in the game. Also, its so easy on the eye, so fast, and so exciting.
I've watched both games, and follow neither, but rugby is by far the better game to watch in my opinion. American football is too stop start, and theres no real spontaneity to the game. For the casual fan I think rugby is much easier to pick up aswell.