I like both but NFL is more complex and tactical like people have already said. It also is apples vs oranges in the sense that NFL is a sprint/burst sport while rugby requires far more endurance. I would say in the past NFL players would have been likelier to get concussions/blunt trauma due to the faster sprint speeds going into collisions but the NFL has made a lot more things illegal, making both sports closer to equivalent in terms of violence. The ads suck but you can watch ad free condensed games (without all stoppages of play and ads) online an hour or two after the game finishes.
EDIT: one thing to mention also is that the NFL has incorporated a rugby element in recent years, allowing blockers to essentially continue pushing through the defense with the ball during a running play for an extended period of time until forward momentum stops, kind of like what you would see in a scrum. In the past, NFL refs would stop a play before allowing a team to continue pushing after initial contact is made, provided the ball carrier hasn't gone to ground.