Boris Johnson, according to a poll, is the person with whom most Londoners would like to have breakfast. Last Friday, I had mine with Ken at a café in Hampstead, after which I had lunch at City Hall with the popular mayor himself.

The Ken-Boris contest used to boil down to policy v personality. It was Ken, though, who turned out to be an interviewer's dream -- indiscreet, careless, outspoken and funny. Box Office Boris has been banished for the duration of the electoral campaign, replaced by an on-message Conservative politician and media-savvy former journalist, painfully conscious of how he will appear in print and of how his comments will be reported.

For such a notorious maverick, I found Boris maddeningly cautious and unforthcoming and it made him less fun to interview than I had anticipated.

The fact that he knows every journalistic trick, after a lifetime spent working as a hack, may well turn out to be one of Boris's greatest advantages over Ken. He is constantly vigilant, on the lookout for the tripwire that will land him in trouble. Once a quote-jukebox, he answered my questions with non-sequiturs, flattery, prevarication, diversion and digression. He even "pleaded the Fifth" to avoid answering in a way that would give the papers a headline. He stuck frustratingly to his mantra about his achievements and plans.

Boris's dilemma is that what makes him popular is also what stops him from being taken seriously as a politician. It's the likeability factor, though, that got him elected. Boris has always been bafflingly bullet proof. Every past gaffe and glitch has propelled Bounce-Back Boris on to greater glory. The public don't seem to care. In a superficial age, he has always seemed fun, telegenic, the anti-politician. He has by far the highest approval rating of any Tory.

Whereas once Ken was seen as competent and Boris charismatic, I found Ken candid and Boris canny. In the hour I spent with him, Ken told me the following: the BBC director general, Mark Thompson, is "a moral imbecile" for vetoing the word "Palestine" in a protest song; bankers' bonuses are "like penis extensions, among a smaller league of men - mine is bigger than yours"; Margaret Thatcher was "clinically insane" when in power; the Daily Mail "has done an awful lot for making us a more embittered people". Henry Kissinger "wasn't going to get laid until he was powerful, you know".

Spoiling for a fight, he was irrepressibly controversial, despite his press officer's best efforts. It's no surprise that he called his memoir You Can't Say That. After all this is the man who called an Evening Standard journalist, who was Jewish, a "German war criminal", for which he refused to apologise and was given a one-month suspension for bringing the office of mayor into disrepute.

Ken talked more about Boris than Boris talked about Boris -- and even though he usually doesn't talk about his children, he couldn't resist a dig: "We both have five, I can admit to all mine." He was referring to his rival's rumoured illegitimate two-year-old.

What did surprise me about Ken was that for someone so politically intransigent, he was open about regrets and failings in his personal life. I got a glimpse of what this self-confessed "workaholic", currently without a job, who still wants to change the world but who admits that "politics takes over your life", is missing. "No one would ever want to marry a politician," he tells me. I know I say. And I do.

It's hard to imagine Ken ever switching off from politics. He is most alive when he's talking figures, policies, plans. And despite once saying that a mayor should serve only two terms, his plans seem to extend well beyond the next four years. Even while gardening, he told me, he talks through issues to himself out loud.

I have heard people say often that they like Boris but they worry about his ability to run London. I worry about Ken's ability to work with a Tory government in a time of economic crisis. For him, the office of mayor is not just about London: it's about changing the system.

Ken is a revolutionary, by nature. That worries me. It's also why I like him.