Interesting topic, I've always wondered the same, there has always been less discussion of Angel, even though most fans have watched both.
I've watched both.
I have a weird thing, were I think Angel is a better series then Buffy in a lot of cases, but there's something about Buffy that makes it more...more, ya know?
I've never been a big fan of discussing episode by episode, I prefer to discuss overall themes or issues, which tend to span more then a single episode. I find it more difficult with shows like Buffy, Angel, Supernatural etc, which have really broad stories, compared to, say like SHIELD which has relied more on an episode by episode feel (at the moment), which I think makes it easier to to discuss episode by episode.
I think it's the mythology as well, both Angel and BtVS have a lot of mythology, but I think BtVS's is a juicier mythology to sink your teeth into for discussions. The whole slayer line and the history of it, and how it changes over the course of the series is so interesting to me.
I think BtVS also had worse character development. Like team Angel are a group of grown ups who make mistake but manage to work together well.
There are a lot more holes, contradictions and and history between the characters actions on BtVS, which allows you to get into some really gritty discussions of the characters ( for example; holes in the plot, can't just be holes in the plot, then end up being part of that character).
I think BtVS is also a little more subjective, like art, people can see what they see in in, but others wont. I think Angel is a more linear story of a main character who has done wrong and is looking to redeem that, whereas the scoobies are in the middle of their mistakes - which is not to put down Angels storylines, he's one of my favourite characters and I think Angel is a better show, but there's something about Buffy!