I'm a fan of both shows.
I watched Xena when it was on TV, but I think it was in reruns even then and I didn't see it straight-through and in order like I saw Buffy, so my memory is fuzzy. I remember that Xena was often on during the weekends. My mom didn't like it, so I watched it with my dad.
Just as Xena is a descendant of Wonder Woman, Buffy is a descendant of Xena. I do not think a TV series about a rather girly super hero would have been greenlit had not Xena's huge fanbase proven that the concept of a female super hero was profitable. In terms of feminism, then, I think Buffy "wins," but only because Buffy was able to take the female super hero to the next level. Xena, you see, was a spin-off of Hercules. For all intents and purposes, she was the female version of Hercules. She was a female warlord who copied her male contemporaries in personality, fighting style, and leadership style. Xena was a strong woman, but she lived in a man's world and she played by man's rules. Xena was a warrior who also happened to be a woman. Conversely, Buffy was a woman who happened to be a warrior. She did not copy her male contemporaries but, rather, reflected her own female contemporaries in personality, leadership style, and fighting style. I don't think Buffy, which I believe began to be popular at around the time that Xena went off the air, would have been possible had Xena not come first.
Just because I think Buffy reflects feminism that is strong because it is not based on making women like men does not mean that I think Xena is somehow lacking in importance as a feminist text. The two characters/shows have a lot in common: the characters have special callings and abilities; the characters fight fantasy creatures and characters that are usually metaphorical in nature; the characters struggle with good and evil internally and externally; the characters lead armies; the characters avert apocalypse; the characters face Heaven and Hell; the characters form close, familial bonds with blood kin and trusted, quirky friends alike; the characters deal with death and resurrection; the characters have foils (Faith and Callisto); and, yes, the characters deal with romance. In fact, they both have a penchant for redeemable evil-turned-good supernatural guys who wield swords and wear leather. The list goes on and on. Both shows feature funny, self-reflexive and culturally aware dialogue. In some ways, Xena surpasses Buffy as a feminist text. For example, the title character deals more directly with issues of motherhood. Also, the Tara/Willow connection probably owes a great deal to the lesbian undercurrents first broached in Xena. There is no denying that both shows are successful and meaningful feminist texts.
There is a lot to compare and contrast. In my view, though, Buffy is clearly a descendant of Xena.