I think that it's pretty much an apples and oranges thing, hard to really compare the two. Yes, in Season 6, Dark Willow made an excellent match for Buffy, but it has to be remembered just how that fight went down. Buffy was pretty much fighting as a holding tactic to allow Jonathan and Andrew to escape. As a result, she had to ignore her greatest tactic, the same one that she eventually used to beat Caleb before she recovered the Scythe: speed. She couldn't, as she advised the Potentials later, "run away. Seek the higher ground. Make the fight your own." She HAD to stay and take a beating in order to allow the others time to escape. Also, it must be remembered that she still considered Willow a close friend and was almost certainly holding back a bit, even if only unconsciously. The results of that single fight shouldn't be the only thing considered in answering the question of who is the most powerful, or skillful, fighter.
Something else to consider is that Dark Willow's power was the result of a magic spell, therefore dependent on her magical energy. After a few minutes of fighting, her power diminished, and she had to recharge, something that Buffy would have never allowed her the time to do under normal circumstances. Buffy would have kept up a "punch and dodge, kick and duck" strategy until Dark Willow's power was completely depleted.
Under different circumstances, with nothing to consider except the fight itself, I think that Buffy would have beaten Dark Willow. It might have been close, but Dark Willow, relying on just her superior strength, would have fallen victim to Buffy's greater speed, endurance, and experience.