They would kill each other, and Western Civilization would win.
Freak BUFFY out? Buffy, the person who fights face-changing Vampires, shape-changing demons, killer dummies (which give her the heebies), and many other truly strange things without batting an eye…THAT Buffy?
It’d take a lot more than a little silly yelling and Hong Kong action flick wirework to freak her out.
I can’t believe I have an opinion on this but here goes. A couple of friends (uber-geeks) got into this arguement at one point a few weeks ago. After a short while they hit the fan sites for stats and figures. Their final conclusion: Buffy hands down. Apparently from official sources they came up with something similar to this.
Strength
Buffy- Apparently she can lift 10 tons in a pinch. This was the hardest to swallow but apparently is used as an official figure by the writers. They however are not supposed to show her doing it frequently or blatently. This is also supposedly confirmed by dialog.
Xena- Near human maximum. Xena is quite strong but has been beaten in feats of strength by normal men.
Edge: Buffy
Agility
Buffy- Buffy can be quite agile when the situation calls for it. She has been known to catch thrown weapons, dodge bullets etc. She does get hit frequently but given her normally not human opponents this may not be indicitive of anything.
Xena- Xena can be quite agile when the situation calls for it. She has been known to catch arrows, dodge weapons etc. Xena never gets hit except by bare arms/legs (no weapons).
Edge: Even (or slightly Xena)
Toughness
Buffy- Buffy takes a licking and keeps on ticking. Literally it is nearly impossible to keep this girl down. Given that most of the beings she fights pack a heavy punch this is impressive.
Xena- Xena also takes a beating well. However she seems rarely able to take a direct hit from a supernatural force and bounce back as quickly as Buffy can.
Edge: Buffy
Martial skills
Buffy- Buffy holds the power of the slayer. This gives her instictive combat skills. She has been shown to be able to use a great variety of weapons almost without thought. She routinely defeats foes who should overwhelm anyone with ease.
Xena- Is a master of weapons. However she is shown regularly to be only a step above other top warriors.
Edge: Buffy
Tactics
Buffy- Buffy relies on the Scooby Gang to provide her with tactics. Without them she tends towards straightforward attacks with little planning or foresight.
Xena- Xena was trained by the god of war. She slips easily into any situation and is able to direct almost any encounter on her terms.
Edge: Xena
Crew
Buffy- With the Scooby gang almost anything is possible. Willow alone might be able to take out Xena. Once the Scoobys are pissed their abilities and power levels seem to rise exponentially. With Giles providing tactics they become a strong cohesive whole.
Xena- With the exception of Gabriel, the other characters in Xena’s world are incompetent. Gabriel would no doubt beat Xander but in the end can’t compensate for the whole gang.
Edge: Buffy
Comments?
I will admit that I don’t watch “B:TVS”, so when this question comes up all I can do is picture Sarah Michelle Geller’s toastrack body and think that even I could mop the floor with her. But I think many of those on Buffy’s side here are similarly unfamiliar with Xena.
In early episodes of “X:WP” Xena did focus primarily on mortal foes – the “warlord of the week”. But as the series progressed her powers were expanded. I hate to bring this up, as I think this sort of thing ruined what was oriignally a perfectly decent action show, but it does make Xena look better when it comes to these arguments. 
First of all, Xena has defeated any number of supernatural foes. Bacchae, banshees, harpies, giants, ghosts, demons, and gods. The woman is personally responsible for the deaths of most of the Greek pantheon. As I said, I don’t watch “B:TVS”, but I doubt she’s managed mass deicide. She probably hasn’t turned an army of thousands to stone either.
Secondly, many have mentioned that Buffy’s “crew” is better than Xena’s. While I would hesitate to pit a bunch of modern teens against even the most ordinary of ancient peasants, it may be that they could take on, say, Joxer. But Xena doesn’t pick her friends because they are useful in a fight. She doesn’t need help when it comes to fighting. She singlehandedly defeated the Persian cavalry! However, if she did feel the need to call in for backup, it is worth remebering that her best friend after Gabrielle is the mighty Hercules. I think she could also rely on the support of the entire Amazon Nation, and they fight pretty good for a bunch of skinny chicks in feathered bikinis.
Third, Xena is a master of not only every conventional weapon of her time period, she also has a Jackie Chan-like ability to use anything as a weapon. She can cause GBH with a frying pan, a piece of cloth, raw fish, even her own hair.
Fourth, I think some of you have underestimated Xena’s toughness. She kept fighting while in labor!
Finally, Xena is an inventor, a brilliant tactician, and (as was demonstrated in the episode “Been There, Done That”) a mathematical genius. So take that, Buffy.
Second episode of BtvS - at the Bronze, vamp has Xander, Buff’s to far to stake him, grabs a cymbal, and beheads the nasty with that. Anything at hand is a handy weapon for The Slayer.
She’s similarly skilled with pretty much every weapon that was period to the middle ages (Slayers apparently don’t update their arsenal very often…), and her body’s pretty much a lethal weapon in itself. (Lethal to any mortal easily, many supernatural foes bite it pretty well just from a hammering, too.)
Lamia, Buffy in the episode where her last lover left her, took on ten vampires at once, and dusted them in 11 seconds.
One factor favoring Xena is beat experience. Xena does have it in spades. As Buffy may sock her a couple of times, Xena will use her wiles to find a way to at least neutalize her, maybe by using those nerve pinches
The big, big edge Buffy has is her supporting staff, the best since Tuxedo Mask in Sailor Moon. Willow is getting strong enough to take on Gods and wind up with the upper hand. Tara is a very powerful enchantress. Xander is quite resourceful when he doesn’t whine about being normal. Spike can’t hit Xena (that little chip), but he can rally someone else as well as anyone. And then there is Giles; who knows what he will resort to?
Xena can rely on Gabrielle, who is almost as smart as Willow and strong fighter herself, and Joxer; she can’t be sure about Aphrodite and Ares.
If Xena uses magic along with her combat, then she might have a better chance.
How about my suggestion: settling it all on X-Files where Lucy will play a recurring character?
Let’s also remember that, if she learns to properly harness it (or, hell, maybe even if she doesn’t), Buffy can call upon the experience of all the previous slayers. (That is what all that “spirit of the first slayer” stuff was about, right?)
I’d win! That’d be one hellva fun fight to watch!
Who cares? As long as there is the obligatory ripping off of clothes we all win!!!