If the question is, "Which Joss Whedon show is the best?" the answer, or at least my answer, is: Neither.
Firefly, as conceived by JW hit the broadcast airwaves full speed, and only Fox's nonsupport stopped it. From the opening two-hour pilot ("Serenity") he and his actors knew what their characters were about, about their individual stories, and how they related to one another and their universe.
And when it came to an end you could see River Tam's damaged genius as maybe the finest creation of the writer who'd previously given us Willow Rosenberg, Dru and Winifred Burkle.
There was never any sense of a show figuring itself out and finally hitting its stride (as Buffy did finally in season three). This was some of the finest television ever. IMO.
Firefly lives!