I voted for two-parters on Buffy because the are simply more good two-parters than arcs, and they tend to be some of the best episodes either show has to offer.
'Surprise/Innocence', Becoming pt 1/2, 'Bad Girls/Consequences', 'Graduation Pt 1/2', 'This Year's Girl'/Who Are You', 'Bargaining' I think the writers are able to get the pacing right in two-parters, to expand on important storylines such as turns to the dark side or season climaxes.
Stand-alones are okay but arcs on Buffy tend to have many ups and downs in them, some episodes are really good and some are dead weight -such as 'Weight Of The World' in the big Glory showdown arc of S5, or 'Two T o Go' between the brutal 'Villains' and gripping 'Grave' of S6
On Angel though I'd have to say that the arcs are much better- Angel going dark, the Birth of Connor, Connor's return, the Apocalypse arc from S4 (which is actually 3 arcs joined together 'the Beast' arc, 'the Angelus' arc and then 'the Faith' arc)
Actually I can't think of any two-parters on Angel? Maybe the Illyria thing in S5.