I'm not gonna choose because I believe that they're all just one big interconnected show anyway, but I'd like to make the following points.
- Angel had greater consistency than Buffy, but didn't reach the same heights. Buffy could have a "Once More With Feeling" one week, then a few episodes later a "Wrecked". Angel was more solid, with no episodes qualifying as awful and very few sub-par, but didn't have many outstanding episodes either. Buffy reached a higher peak of awesomeness (S2+3) but also a lower depth of crappiness (S6).
- Angel owes much of its success to Buffy - just look at the list of major characters. Angel, Cordelia, Wesley, Spike, Harmony and Darla all started off on Buffy. However, I think Angel improved most of these characters, in particular Wesley, Angel and Darla.
- In my opinion, Angel had better romances for its lead. I preferred both Cangel and (if Darla's human) Dangel to Bangel, Briley and Spuffy. However, I think Buffy had better romances for its other lead characters - I like Willow/Tara, Xander/Anya, Willow/Oz and Xander/Cordy, but Fred/Wesley and Fred/Gunn bored me (I did like Wesley/Lilah though).
- I preferred Buffy's system with having a different Big Bad each year as apposed to Angel, which for the first three seasons at least just had Wolfram and Hart as a constant Big Bad, and the finales would often feature some unrelated threat. However, I did like W&H and Buffy would sometimes end up with a rather lame Big Bad (eg. Adam and the Trio).