Kramer vs. Mad Max
That's like asking which you'd rather have, colon cancer or prostate cancer. Both are a huge pain in the ass and you're better off having nothing to do with either of them.
Materially, there's not much difference in their slurs. Far from being mere slips of the tongue, these are world-class agony of defeat tumbles down the slope of human dignity. That their malodorous utterances are jarring to the sensibilities of all decent people only scratches the surface.
Gibson, by invoking the Hitleresque trope of the Jew as instigator, and Richards, with his fond remembrance of what "we" would have done to these two black men "50 years ago", are both exploiting prejudices that extend back centuries. The Jew is a parasite, sowing discord and feeding off of an embattled Christian society. The Negro's fate is bound intrinsically to the whim of the white mob with a length of rope and the law on their side.
The difference, then, comes in their relative stature as celebrities, and in the relative ascendancy of their "ideas". Michael Richards is a has-been. Certainly, he was a big star on one of the most popular sitcoms of all time, but that went off the air eight years ago. His last acting credit comes from 2000 (although he is slated to voice a character in an animated film set for release in 2008). Suffice it to say it's been some time since Richards has rubbed shoulders with the Hollywood power elite.
Gibson, on the other hand, is at or near the height of his power as a Hollywood player thanks to the massive success of The Passion of the Christ. He's not a major actor any more, but his producing and directing credentials are stronger than ever.
When the Gibson scandal broke, Mel found his fair share of defenders, from those saying his words were just the incoherent rantings of a drunkard to people who thought he might just have been making a point about Israeli military hubris. His sad predicament also elicited a chorus of right-wing voices convinced he was being discriminated against because of Hollywood's anti-Catholic bias (if you don't believe me, follow the Mel Gibson link above).
It's highly unlikely that Richards will get the same treatment following his racist rant. He has no natural base of supporters and, frankly, ugly anti-black bigotry just isn't as hip or acceptable as antisemitism is in today's world. Many Americans may harbor such ugly feelings in their hearts, but they would never say so out loud.
The final point of consideration involves delving into the history of these two buffoons. To my knowledge, Richards has never before come undone like he did at the Laugh Factory. It doesn't mean he hasn't been a racist all his life—such pronouncements rarely spring from the untainted mind—but he has no public record of such behavior.
Gibson, on the other hand, has long been a suspected antisemite. He belongs to a Catholic sect known for its demonization of the Jews, he refuses to directly disown the Holocaust denial of his father, and he came under fire for the perceived antisemitism in The Passion. His freakout fits neatly into a pattern of antisemitism and his evident lack of self control.
Putting all this together, I arrive at the conclusion that Mel Gibson is worse. That said, I wouldn't invite either of these jackasses over for dinner.