when you think about it. On its face, the Incognito voicemail sounds much worse. However, put in context, the Riley Cooper thing is worse.
With the Incognito voicemail, the circumstances are such that it leads to the inescapable conclusion that it was a joke, even if it was an inappropriate joke. While it certainly doesn't excuse the use of racial language, it changes the conclusion from him being an actual racist asshole to him just being a dumbass. There's already been stuff that came out that Martin treated it as a joke at first, that he was the one that passed it around to teammates, etc. For all we know, he could have jokingly referred to himself as a "half-n****" and it became a running gag amongst the guys. The Incognito thing strikes me almost like he was doing a parody of what an actual racist asshole would say.
With the Riley Cooper thing, he was speaking his mind, feeling comfortable in the company of all his white friends at a country music concert, and not realizing he was being filmed, so you were basically seeing his true character come out.