I think we've had this conversation before. I think American football has bigger hits - more powerful players that are trained for explosive short bursts of activity with the protection of pads and helmets. Most American football players wouldn't make it to the end of a rugby game though as it is a full on 80 minutes of activity, plus the big American football players wouldn't be mobile enough on the field and the mobile American football players wouldn't be big enough to be effective. These are generalisations of course, and I am sure there are some players in either code that would be able to cross over.