Most of the anti-AGW crowd is simply doing armchair, a-priori reasoning behind why AGW is false. "Humans are too puny to have an effect!" they say, or "The climate has changed drastically fast even without humanity being around!" Often there are political reasons for holding this position--certain arguments on how to deal with GW are certainly political in nature, and may come into conflict with one's own dogma, and thus psychologically one may be predisposed to oppose GW on that basis.
HOWEVER, that does not mean that some people that argue for AGW do not fit into the same shoes. Remember, just because you are "correct" does not mean your reasoning is. Naturally, someone that hates big business and "the man" may also psychologically have a reason to believe in AGW--another reason to rage on about the status quo.
If I was a betting man I'd bet for AGW, but really I know the science behind it is quite complicated and I know I'm nowhere near competent to make a good, solid argument on the matter, so I must approach the issue with a tempered agnosticism while leaning a bit towards the AGW side because that's the verdict by a vast majority of hard-working PhDs, and I highly doubt that climatologists consist of some dark, left-wing communist sect of economy-destroying conspirators. That is what true skepticism is, noncommitance (particularly emotionally) to a position particularly when you are not an expert on it. Many on both sides of the GW debate are not skeptics but reactionaries with their thought ruled by political underpinnings. Most of the people I know that rant about how AGW is a fraud no absolutely nothing about the mechanisms scientists go about acquiring the data on past climate conditions.
"The algorithm to do that is extremely nasty. You might want to mug someone with it." -- M. Devine, Computer Science 340