I've played both, and it's comparing apples to oranges. I prefer rugby. As a guy who is my size, I find it easier to play Offensive Tackle or Right Tackle, then I do playing Prop. It's a totally different ball game. Are most NFL line backers, stronger than S14/Aviva Premiership Props? Of course they are. Steroids have been a common problem in the NFL for 50 years, and the NFL players don't need to be able to last 40 minutes on the field at a time, they need to last 40 seconds. The biggest, fattest person can manage that. As for rugby, being a big guy myself, you play a high intensity match when the ball is in play 4-5 minutes at a time, and there are guys 15-20kgs lighter than you, that you have to beat to the break down, and guys who are even lighter than that, who you have to stop from getting past you, it's bloody tireing.
Who are harder hitters? Well, probably most NFL players, as they are padded up to high hevan, so they don't kill each other, and some people would die if they weren't padded up like they are. They hit using the sholder, and while it happens in League, it doesn't happen when the people hitting are 310lbs. The runners also have a lot to do with it, as you may notice that many receivers in the NFL often jump up high, to avoid low tackles. If any of them did that without paddings, that is certainly a broken neck.
It's very much a different type of toughness. Rugby requires constant work, so all players have to be fitter while working very hard. NFL players have to be extremely good at what every specific area of the game they play. They don't play for very long, and they usually will only be able to play in their specific position. I think it's like comparing a weight lifter who can dead lift 800lbs for one rep, against a weightlifter who can dead lift 550lbs for 5 reps.
