In terms of contrast with a hypothetical pair extremely-candid candidates, (rather than the equivocators that actual politicians have to be) Obama and Romney's biggest errors (in my opinion) are these:
1. Obama failed to acknowledge that despite the "terror" label in his initial post-attack speech, he still took his time, in most other contexts, in labeling it a terrorist attack. The Daily Show had even highlighted this reluctance of Obama's. It's not a biggie in my opinion because it's really just a label either way -- I'm reminded of how a number of conservatives insisted that the Fort Hood shootings be called a terrorist attack, and one clever counterpoint from some commenter was along the lines of "Okay, so now it's a terrorist attack. Exactly what do we do differently?"
2. Obama's "5 million jobs" number is really cherry-picking from a particular portion of time; it's rather like how global-warming denialists liked to say "Global warming stopped in 1998" because 1998 had been the hottest year on record, ignoring the obvious trend up to that year and after it. I forgive it because it's not a "lie" lie and he needs to be able to say something to prevent Romney from winning ("hundreds of thousands of jobs" doesn't cut it). But I'm surprised Romney hasn't countered it, just as I'm surprised he didn't have a counter to Obama's point about the Benghazi attack.
3. Romney wins for the most blatant, straightforward "lie" lie. You don't have to contrast a hypothetical hyper-honest Romney to see that he's saying something he ought to know is false. The lie in question?
"I know he keeps saying, `You want to take Detroit bankrupt.' Well, the president took Detroit bankrupt. You took General Motors bankrupt. You took Chrysler bankrupt. So when you say that I wanted to take the auto industry bankrupt, you actually did. And I think it's important to know that that was a process that was necessary to get those companies back on their feet, so they could start hiring more people. That was precisely what I recommended and ultimately what happened."
The key term here is "take". The truth is that no one has ever accused Romney of wanting to "take Detroit bankrupt". That would make no sense, because Romney explicitly did not want the government to "take" Detroit at all. The actual accusation was that Romney wanted to let Detroit go bankrupt. Where did that wording come from? Well, from the infamously liberal New York Times, and the headline of an editorial, "Let Detroit Go Bankrupt", which puts crazy words into Romney's mouth about not bailing out the car companies by a single penny. (The author of the piece in question was some governor named Mitt Romney.)
So where does Romney get off saying that "ultimately what happened" was "precisely what [he] reccommended"? It wasn't, and it would be nice (or at least refreshing) for him to actually own his prior semi-libertarian stance. But he can't do something like that, and I'm starting to develop a hypothesis (I actually just thought of it while composing this) as to why.
I think Romney's attitude towards his own prior positions is like that of most Christians towards the Bible. To some, this may sound strange, because don't Christians treat the Bible as God's perfect unchanging word, whereas Mittens blows with the wind? Well, my view is that both Christians (in a very large, centuries-and-continents-spanning way) have blown with the wind rather like Romney has. They (at least Protestants) claim that the Bible comes first, informing them of their views.
In principle, a Martian could derive the views of modern Christians by reading Scripture cover-to-cover. In practice, ha-ha-ha. You've got all the areas in which the text contradicts itself (hey, sound like any Republican presidential candidates you know?). You've got the explicit overrullings of previous instructions, such as the repeal of kosher, but that's the least of your worries. Most signifigantly, there's the apprent endorsement of practices with which modern Christians strongly disagree. Two notable examples would be slavery and polygamy. A weak example is abortion, which does not get condemned the way conservative Christians would condemn it. In any case, even a partial derivation of the Christian Right's philosophy is impossible.
So what do Christians say? Well, some of them admit that things have changed -- not that God has changed, but that the Bible is simply a less-than-perfect documentation of ancient folks' beliefs. But most of them don't take that mostly-honest route. Instead, they simply superimpose moral views which have modern consensus, like "Slavery is wrong", onto the text. Of course, a loose enough reading will allow for any interpretation necessary. So voila! Where the atheist's syllogism is "Slavery is wrong, the Bible tolerates it, therefore the Bible is wrong", the Christian simply takes different premises to be more fundamental: "Slavery is wrong, the Bible is never wrong, therefore the Bible doesn't tolerate slavery." Repeat for genocide, subjugation of women, and polygamy.
So what does all that have to do with Mitt Romney? Well, one of his lies from the first debate was "pre-existing conditions are covered under [his] health care plan". There's no real sense in which they are, rather like how there's no real sense in which the Bible says no to polygamy and yes to monogamy. But as of 2012, laws mandating coverage for pre-existing conditions are popular, and polygamy is passe. Thus, ergo, therefore... the Bible frowns on polygamy, and Romneycare deals with pre-existing conditions. Quod erot demonstratum.