This is what I wrote on another forum.
I personally prefer Ats but I've always thought Btvs was more popular.
It's not just about having a female lead because it's very possible to have a poorly written lead. I think it's that a female lead ties in with almost subverting what is 'established'. Buffy is this comptempory figure overthrowing the ancient and status quo.
I also tend believe the characters are more accessible on btvs, or more fun. I often talked to many 'part-time fans and most of them prefer the Xanders, Buffys and Spikes to the Wesleys, Angels etc. As a quick reference people would recognise Xander and Spike as funny and/or cool, but Angel as boring and always brooding.
I tend believe that the episodes are more accessible as well. Although both are arc heavy, the average person can pick an episode of Btvs (even the not so creative ones) and enjoy it on the merit of the episode itself without necessarily 'getting' Buffy. With Ats, I personally found my enjoyment of the show was more to do with an appreciation of the overall story, but in point of fact that is why I've enjoyed it more. I found more overall story and character satisfaction, hence why Angel, Wes and Faith are my faves.
So I agree that btvs has more 'accessible' attention getters.
I actually like that Ats didn't have one single clear message, because for some reason, that resonated with me more. I suppose this is why I've always haboured an affinity with Btvs season six, because it was kind of 'systematically unsystematic'.
To me Btvs felt sort of like it went full circle (as far as Tv series is concerned) and Ats kind of spiralled outwardly which of course never bothered me. If one show is about certain aspects of life then the other simply engenders another side and while Joss stated that to him Btvs is about life itself, I think that can extend to the Buffyverse in general, sometimes the journey can lead you farther than you expected and ideas can blossom in the most organic of senses. This is why, personally, Ats will always be closer to my heart.