Well, I haven't played American football although I have seen it a few times on TV, on the other hand I could talk to you from my perspective as a former rugby player, I think American football is a bit more aggressive, but that's why american football players wear some protection, not in rugby, I have been tackled while suspended in the air (which is considered a foul) by people weighing more than 100 kgs and take quite strong blows playing rugby (at that time I would be around 80 kgs of weight) also in the moments of rucks and mauls, I have come to have 5 people of more than 80kgs on top of me for a few seconds which was not pleasant at all, and much more as you can imagine, they are two quite aggressive sports, but you get used to it over time