i dont have a bias either way but its like this.
BOTH sports go as hard as they are allowed in order to achieve the game's objectives.
in NFL u have protection and helmets and as a result u see really full on hits... the guys are out there for a fraction of the time of a game and hardly anyone touches the ball.
rugby tackles are a trade off between hurting and getting hurt..they stay on the field the whole time, and have the ball many times throughout the game.
IF NFL had one team that did all of the plays, then you could compare it to Rugby as theyd get more exposure to different aspects..but they dont.
still, without a doubt, the most athletic game on the planet is australian rules football..hands, feet, eyes, hits, tackles, duration, accuracy, team work, sprint, endurance.
olympic calibre athletes have been lost to AFL for decades now.