I laced up the skates since I was 3 and played juniors (semi-pro) in Russia, Canada and Sweden to name a few. I can't knock the conditioning of a pro football player because of the different positions and demands that a football player encounters. However, here's my comparison:
Line: 3 feet away, slapping, pushing, eye gouging, top speed within that distance is about 8mph
Corners/Safety/Linebackers: Running starts, sometimes hitting players blind (new rules preventing defenseless hits) Top speed approx 18mph (figuring 11 seconds/100 yards....300 feet divided by 5280 which is .057 miles at 11 seconds)
Hockey: You're a bad ass muthafucka! End post.
Just kidding, but overall, even a goalie's conditioning has to be spot on with the quick jolts, twists and turns. Top speed for a hockey player I believe is 33 or 34mph so even if you figure average speed around 24-27mph, that's a lot faster than a top speed cornerback (and most likely the hockey player would be bigger than a corner or safety)
I played both but call me biased.....Hockey is a tougher sport and pound for pound, I'd take a hockey player over a football player with their toughness. However in a fight, probably 80% of football players are carrying a gun so they win there.....in closing, Not many football players are coming back in the 4th quarter with stiches over their eye or a broken finger.
Hockey gets my vote on this one