BTVS was always the more popular and ground-breaking show, plus spin-offs are rarely as popular as the original. Still, I've come to enjoy "Angel" more for the following reasons:
1. More interesting main character. 240-year-old vampire cursed with a soul on a quest for redemption is more interesting than "hot blonde woman deals with frustrations in school and in her love life"
2. Better setting. Supernatural detective agency vs. high school, college, or wherever. Plus L.A. vs Sunnydale
3. Wolfram and Hart as series-long over-arching antagonist vs villains of the season on BTVS
4. Angel is more adult and darker in tone. By avoiding the high school setting, Angel was able to tell more adult-relevant stories, plus avoided the drift from a mid-series shift in settings after graduation from high school on BTVS. Angel was also grittier and more noirish in tone, with a darker message of the "no big victories" and that the fight against evil is always going on
5. Angel at its weakest(perhaps season 3 or 4 depending on your view) was better than BTVS at its weakest (season 6)
This is all of course just IMHO. And I will say that BTVS probably has the better supporting cast(Giles, Willow, Xander, Oz, Tara, etc. vs. Gunn, Fred, and Cordelia? no contest)