Alright, I'm an American, and I will be biased here, but I truly believe American Football and Rugby are at the same level for different reasons, and it has to do with energy expenditure.
See, everyone's talking about equipment here like American football players have enough body armor to stop a bullet and rugby players are essentially nude. But that helmet that American football players have? Your head will be bouncing around in it once you get hit by a linebacker. It's also a tad sharp. That armor is so you don't break all your ribs at once.
Because you see, those breaks that are in between plays? Those are for the players to communicate, get their plays going on both offense and defense, create mismatches, etc. They also provide a convenient frame of rest for the players on the field. Rugby players are on the field for a much longer time, but all that means is that they hit less harder each time. Football players don't have to worry about the continuous game; they throw their bodies into each and every hit. It's kind of like boxing, where fighters take small breaks between skirmishes to maintain their power.
And then there's the speed. You can only appreciate NFL speed by going to the games. TV is too slow to handle it. Both offensive and defensive players move at significantly faster speeds than rugby players. It's why NFL teams look at the 40 yard dash so much. Power is only half the equation when you're looking at a hit; velocity is the other half. A fast wrecking ball will do a hell of a lot more damage than a slow wrecking ball. NFL players are extremely fast. If they are on offense, they will score; if they are on defense, they will hit a lot harder or make a play deeper down the field. As I mentioned, this is barely noticeable in the NFL; this is very noticeable in college football, however.
Yeah, rugby players are tough on a consistent basis, but football players have to be tougher on a less regular basis. Sports Science did a analysis on force generated per hit and found that football hits were a lot harder even with the equipment on (mostly because of the speed difference), but averaged out for the # of plays a rugby player makes in a game and a football player makes in a game, the avg force was very similar. Once again, if you think that Rugby requires a lot more stamina, keep in mind that football players must give it their all every play if they want to win. There is no downtime once play begins. It's a different kind of stamina.
Now, away from Rugby, I enjoy American football mostly for the strategy and hometown support. And I really can't stand soccer, it's just too slow. I fucking love hockey, though. That's my favorite sport, easily. That's skill, speed, grace and power in the same package.