It's not opinion tho is it? How can it be, the two sports are mutually exclusive, two athletes taking from the best NFL team and the best football team are at the highest level. I assume you thinking on the lines of: an NFL forward could beat up a footy player so the NFL player is better?!
Well, in the same breath, America... CAN'T participate, because you don't have the athletes.
Yes, this is all a matter of opinion. No, I'm not talking about beating anyone up. That's probably the soccer mentality, where losing could cost you your life.
I'm talking about speed, quickness, agility, strength, strategy, and intelligence. You need both to play football and soccer. What we're talking about is degree.
America sucks at soccer internationally because our best athletes don't play soccer. If athletes like Barry Sanders, Deion Sanders, Jerry Rice, Adrian Peterson, LeBron James, Kobe, Chris Paul, Vernon Davis, Michael Vick, Randy Moss, and other athletic freaks devoted themselves to soccer instead of football they would run circles around international players.