Intuitively i would be inclined to believe that if you pick a random pro from each sport and have them compete on a battery of tests that go from strength to speed to endurance, odds are the average rugby player will win, and probably by quite some margin. He might lose a test or two but he wont be terrible at anything.
NFL roles are way more specialized than rugby roles. Even the heaviest prop needs to be able to run for at least 50-60 minutes at a fast-paced pro sport, he needs to know how to tackle, how to pass, how to scrum, etc. Even the tallest and most uncoordinated forward can pick up the ball from a ruck and make a decent pass to the fly half. Some NFL players couldnt pass the ball even if their lives depended on it.
We've all seen wings play as flanker if they needed the extra man on the scrum (YC, RD, etc). Not that they do good, but they can do.
Put a quarterback as a linebacker and see how that goes.
Having said that, i'm pretty sure that for each specific strength and speed test, the best performance will come from an NFL athlete. The fastest 60/100 meters time will most likely come from an NFL player. Same for deadlift, etc.
I guess the biggest difference is endurance.
Now that i think about it, as sevens player will most likely be even better well rounded. It's a no brainer actually.
