I have to say I felt Angel was the darker show, more mature. Maybe because it dealt with adults not school kids, maybe not. I preferred the characters progressions from Buffy in Angel, such as; Faith, Cordy, Wesley, Spike, and of
course
Angel himself. I also totally fell for Doyle's character and was unbelievably gutted in Hero. I adored season one of Angel so much so that almost every episode there were tears and my heart went out to the characters and their development and story-lines. Don't get me wrong, I adore BTVS. But for me it's Angel. I don't know if its because I was (am) an adult when BTVS & Angel aired so watched it as an adult or not. I don't know the core analysis behind why I find I can relate more to Angel the series and enjoyed it more, I just did (and do). I don't think I can compare the 2 shows season by season as we already knew most of the characters in Angel from Buffy. SO don't ask me to do that either. My heart was beating fast for more episodes of Angel than it did Buffy. Besides as a Bangel shipper it had IWRY. As a Faith fan there was 5by5 and Sanctuary. As a Doyle fan there was Hero. As a Cordy fan there was City of..., Room with a view, Parting Gifts, That Vision Thing, etc etc... I could go on for ages (well 5 seasons anyway lol). It's just all my opinion, my view, my perspective.
Note; I'm not a comic reader or a fan of the comics, so I only go by the tv shows.