Buffy the Vampire Slayer got out of the gate first and maintained popular in comic books some time after the demise of her television series. Despite that, Xena-- who premiered on an episode of HERCULES: THE LEGENDARY JOURNEYS before gaining her own series the same year-- is arguably the more ambitious femme formidable.
To say "ambitious" is not to portray the teleseries as something artsy and high-falutin'. XENA the series borrowed from a variety of popular sources-- sword-and-sandal movies, Hong Kong kung-fu films, dungeons-and-dragons and even westerns. Nevertheless, the writers were gutsy enough to simulatenously swipe from highbrow works like Greek epic and the Bible or from philosophers like Schopenhauer, resulting in a unique blend of the "high" and "low" that BUFFY can't quite match.
The theme of the warrior trying to turn his/her deadly skills to good ends is a favorite American theme, but the creators of XENA upped the ante. XENA episodes often concern the necessity for characters to "let go" of the lust for hate or vengeance -- and not only the villains. Both Xena and her sidekick Gabrielle frequently have to practice what they preach, and they don't always do so successfully. The Schopenhauerean ideal of relinquishing the will plays better than it lives.
Not to mention that in addition to all this philosophical complexity, Xena also had better fight-choreography than Buffy--
AND--
A better all-musical episode.
