Angel is more erratic than Buffy as a show. In my opinion. Season 1 of Angel, which I'm very fond of is highly experimental and I like that. 2 and 3 can be hit & miss in places. I love season 4 though it gets slated. And season 5 is incredible! Literally incredible!
---------- Post added at 06:23 PM ---------- Previous post was at 06:20 PM ----------
Angel is much darker and more adult than Buffy. Whilst in my opinion, Buffy the Vampire Slayer is about growing up, ascending from youth to adulthood, Angel is more about grown-ups and the choices they make. And most importantly how they live/deal with those choices.
Angel's preoccupation with the themes of redemption, temptation and freewill make it much more of a morality tale than BTVS. In my opinion. I can't decide which I prefer, a preference is too difficult.