I do not believe that calling a black person a nigger and calling a white person a cracker is even in the same league.
Maybe if the people calling white people cracker had enslaved whites, tortured the men, raped the women, and lynched men women and children for years they wouldbe close, but as it stands now its not even close.
Nigger carries with it a whole host of connotations, its not just an insult, its an implication that african americans are not humans on the same level as the rest of mankind.
Personally when a black person calls me a racial slur, it doesn't bother me in the least, especially when I think its a small price to pay for what my ancestors (from North Carolina, South Carolina, and Georgia where my father's side of the family is from) did to theirs.
Any racial slurs are ignorant and offense, don't get me wrong, but there is something exponentially worse about an oppressor further oppressing a race that hasn't had a fair and equal shot in this nation since they arrived.