Except, of course, that Comey is not the one who decides whether or not to prosecute. He's only making a recommendation to the person who is. That mutes any potential impacts on his "legacy" from how that ultimately shakes out.

If we accept that Comey's primary concern is how he will be remembered in history, then the most logical thing is that he will be remembered more for how well or poorly the FBI did its job than the specific results it came up with.