Well, both cases involving prostitution. One key difference between Spitzer and Vitter is that Vitter is still holding public office [1]:

Sen. David Vitter, R-La., has been mostly mum on the prostitution scandal that forced Democratic New York Gov. Eliot Spitzer to announce his resignation last week. But Vitter let down his guard a bit in a conference call with constituents. Scott Jordan, editor of the Independent Weekly of Lafayette, said he was able to ask Vitter whether he would resign after his phone number was connected last year to a Washington, D.C., escort service that federal investigators say was a call-girl operation. "I have made a very serious mistake a long time ago and I have to live with that every day," Vitter said, according to Jordan's account. "That's not a flippant statement. I need to spend my whole life making up for that." According to Jordan, Vitter turned "a bit defiant" and added: "Anybody who looks at the two cases will see there is an enormous difference between the two of them. The people that are trying to draw comparisons to the two cases are people who've never agreed with me on important issues like immigration and other things."

Last July, CREW filed a Senate Ethics Complaint [1] against Sen. Vitter. We're still waiting for action on that.

Last week, the National Republican Congressional Committee called on Democrats to return contributions from Gov. Spitzer, but no similar call has been made for Republicans to return contributions from Sen. Vitter. Here's a link [1] to a list of those campaigns that have received campaign contributions from Sen. Vitter.