I think a more apt comparison is between Cain and John Edwards in 2008.
Edwards was, like Cain, one of the top polling candidates. But in Edwards' case, only the National Enquirer paid much attention to the affair (which resulted in a child out of wedlock and, ultimately, Edwards being indicted for misusing campaign funds).
The press all but buried the Edwards story until after he was out of the running for president. They jumped on the Cain story while he was on top.
Seems like a double standard.
The National Enquirer did stake outs on Edwards mistress. Cain had women hold press conferences. The tabloid actually went further to get Edwards. So perhaps a double standard but one that favored Cain. Edwards wasn't even able to play the race card.