> Reasonable people can differ only if their prior information is different.

Disagree entirely.

When evaluating complex choices in a chaotic world, we humans use tons of heuristics. There are a lot of things that aren't quantifiable information, but still allow two rational people to reach different conclusions: models of the universe, weight attached to different speakers, moral stances, worldviews, etc.

Tautology. :) People with the same priors cannot either disagree or exist, in the real world.

I think people can only agree to disagree when they can see past their differences. If it's something like which are better, tomatoes or potatoes, then it is easy. If it is something deeper to one's center like PC or Mac, or some programming language then it's harder, and conflict can end up huge if it's over life and death or reputation building. In mostly anonymous and almost random meetings, such as HN, agreeing to disagree is hard, and more often than not, conflict or alliance is expressed through moderation rather than enmity or camaraderie.

At the end of the day, I tend to view argument as vulgar, but put up with it in striving for truth.