My money's on The Highlander. The Slayer's usual weapon of choice is a wooden stake; using bladed weapons only if they're around. The stake will work great against vampires, butnot so much against an immortal comin' after her head with a sword.
But the Slayer does have more strength and can defend herself against an Immortal. I'm still unsure of which I should vote for. Plus, throw a bladed weapon in her hand (ala Season 2 finale vs. Angel) and we got a massive Wrestlemania match.
All the slayer has is superhuman strength. If Buffy had the knowledge of the previous Slayers, why didn't she ever access that knowledge?
Duncan has 400+ years of fighting experiance. Plus thanks to the knowledge gained from the immortals that he slayed, he probably has thousands of years of fighting know how. So the Highlander wins.