American Football is like a chess game; each play is deliberately planned, and each team attempts to anticipate and overcome the other team's move. A single misstep or being out of place by a fraction of an inch can easily spell 6 points for the other team, and cost you the game. To say it's a game of nothing but pure strength is foolish.
Soccer has a lot of flow and technique to it, and for that I can appreciate it. However, there often seems to be too much buildup for little action. Often a team will break away and cause the stadium to erupt which is great but there are just too many dead spaces for me to really be able to enjoy the game. Compare to hockey or rugby (I particularly enjoy watching sevens) which both exhibit the flow of soccer and the physicality of American football.
That said, American football is my favorite sport bar none. Go Pack Go! After that is hockey, then rugby, and I do enjoy soccer but I only really watch it during the World Cup admittedly.