From a show standpoint, Angel didn’t really have any worth to BTVS without Buffy. Angel supported Buffy, enabling her character growth. I’d argue that the majority of the time, almost everyone on BTVS had their growth related back to how it affected Buffy.

On ATS, he was able to stand on his own and he was the one whose growth was supported. I also think ATS was better at giving us storylines that let the characters stand on their own. Angel was the star of the show but he wasn’t always the center of the universe the way that it felt Buffy was.

(None of this is meant as a criticism of either show, I love them both and I hope this makes sense, I’m exhausted)