Another Tool said: Are you kidding?
Trent Reznor's sounds alot deeper than Cash's, the only reason people say that they prefer the Cash version is because he died shortly after it came out, it's your opinion though i guess.
If people wanna hear heaviness, listen to NIN's "Ruiner", the riffs are something else, but the outro is brutal.
I'm not familiar with Hurt (either version), but my general opinion on the Johnny Cash covers is that they are overrated. Sure, I like them, and yes, they do tend to be emotional, and quite good arrangements of familiar songs, but part of their appeal comes from the fact that he sounded so damn unhealthy.
Still, his versions of The Mercy Seat and especially Rusty Cage (waaaay better than the original) are really great.