Hard R or Soft U is one inherently more offensive than the other. Even if you say N-Word or N-Bomb, people know what you're implying, so I think the whole conversation is fairly arbitrary, but I had a thought. The Soft U is a product of Ebonics, so wouldn't a white person saying the Soft U be culturally appropriating African American etymology? I mean, the hard R is always what white people said. It's almost even expected today. That's not even to take into account that the word was in usage before the transatlantic slave trade. But, personally, I don't think there's anything more embarrassing than the jive-talking white man.