Andrew Sullivan notes a key difference:
When he tries to talk of a distinction between Jesus' admonition to help the poor and socialism, he is not nuts; but his excitability and emotional vulnerability get the better of him and his arguments. He is utterly, inexcusably irresponsible a great deal of the time (on Obama and race, for example), but what distinguishes him from O'Reilly or Hannity is his obvious occasional self-doubt, captured hilariously in this 2 am letter to Sarah Palin:
I said: ‘Sarah, I don’t know if I’m doing more harm or more good. I don’t know anymore.’
This is not, I think it's fair to say, a question that has ever occurred to Sarah Palin.
I tend to agree with Andrew on this, although I'm still undecided as to whether Beck is a giant faker or not. My inkling is that he may actually believe the nonsense he spews on a nightly basis so is therefore probably not a bad person. This isn't to say the guy is a serious danger to the population (and probably himself), but his intentions aren't to hurt anyone.
Palin on the other hand, is not only dumb, but mean. She's a narcissistic, vindictive woman who is only able to contemplate politics as it relates to her own success. While Beck has shown an ability to be self aware, Palin has absolutely no idea how poisonous a character she is.
One of the lines the GOP had her parrot in the 2008 election was that Obama was 'palling around with terrorists (he apparently once sat in a room with a man convicted of minor domestic terrorism in the 1960's). The line was outrageous, misleading and clearly a play on Obama's ethnicity. By associating Obama with terrorism, the GOP knew much of the country would believe Obama was secretly a Muslim and an AlQaeda implant. McCain was not called upon to do the dirty work as what was left of his conscience would have stopped him from repeating such nonsense, but the GOP knew Palin was stupid and mean enough to do it, and they sent her out to accuse a US Senator and Presidential candidate of being a terrorist.
This is a line that Beck also parroted, but the difference is that Beck probably believed Obama had a terrorist past, whereas Palin simply didn't care.
This isn't to say that one is worse than the other - both display different symptoms of shameless narcissism, idiocy and delusion, and both are doing extreme damage to the country - but Palin meanness sets her apart, and in my view keeps her in pole position as the country's biggest problem.