"fair fight" was shorthand. They both realize they are going to fight at the same time, with little notice. They are facing each other, and neither has an advantage in position at the time they realize they are going to fight. Neither has an advantage in weapons. Etc. Etc. at tldr. A "fair fight" is unlikely to happen by accident, but an enemy might set one up.

I just made my reservations for F2F in Denver so everyone here should do that too. We can watch SPN dvds and squeal in person!

Why would an enemy set up a fair fight? Why would anyone set up a fair fight? Why would you bother fighting? If you have the luxury to do all that, you're not fighting, you're sparring.

An enemy might set up a "fair fight" between the brothers as part of an incoherent overly complex plot.

So far the enemies have done anything but, with the demon blood and the insanity of the siren infection. Unless we're going to get a Fight Club episode (tired!) I can't see it.

Heck, I think the Siren ended up accidentally creating a "fair fight". In general an evil son-of-bitch might set up a "fair fight" between the brothers for maximum emotional pain, then kill the survivor. "fair fight" so that the fight would last as long as possible.

And we are not really talking the plausibility of a fight happening. We are talking about who would win if it happens. I stick by my answer though maybe the term "fair fight" carries baggage I don't intend.

If the fight happened with no advantage to one or the other except those advantages and disadvantages that go with being who they are Sam wins - probably. Assuming equal notice, no weapons difference, no positional advantage to anyone.

If Dean has even a slight advantage he wins.

How was the siren fight fair? Dean gets the drop on Sam with a knife, he knows about the fight first since Sam isn't infected right away, and then later goes at Sam with an axe. Sam doesn't have any weapons.

I don't think I get what you mean by fair. Which is why I don't believe they're applicable outside of sports, with rules and bells and weight classes. Not in actual fights.

Lets leave aside "fair". Sam and Dean are subject to a spell that forces them to fight. They are facing each other when the urge to fight happen at the same time. No weapons or the same weapons. No advantage of position. Sam wins. Body mass and muscle. Dean's greater experience, and I think greater talent for combat does him no good.

Give Dean an advantage - more time to plan than Sam, better weapons, better position and the that same experience and perhaps talent can be deployed to compensate for Sam's greater strength and mass.

I mean you can say "oh no, the first case will never happen". But if you are going to question who will win in a fight between them then it is not unreasonable to think of different circumstances. And equal weapons, no great positional advantage for one or the other, equal time to plan is certainly one set of circumstances. So forget the term "fair fight". Too much baggage I don't intend.

Another way to look at it is Sam is stronger more muscle, more mass. If combat happens in circumstances where Dean has little chance to compensate for that, Sam wins. If Dean is given a chance to compensate for Sam's greater weight and amss then Dean wins, because Dean has more experience and more talent for combat.

You know what this discussion reminds me of?

Look, if cavemen and astronauts got into a fight, who would win?