I've always thought that these two seasons, along with AtS Season 5 and seasons 2/3 of Buffy, were the best the Buffyverse had to offer, despite a few flaws each of them had. Between these two, it's really difficult for me to choose.
I think season 5 was a little more consistently good, with the only things dragging it down for me being the stupidity of the VAMPIRE WHORES arc, and, as usual, Spike being there for no apparent reason outside of fanservice (admittedly, though, "Fool For Love" is awesome). But otherwise, this is Buffy and the Scoobies at their best, matured after their teen years but not yet the obnoxious, cynical adults they would further grow/regress into, and united as a family against a common threat. This was the season that best utilized Dawn, and while she wasn't the best Big Bad the series had to offer, I loved Glory. Really, if they added return cameos from Oz and Cordelia, this would have been the ideal season to end the show.
The Darla arc on Angel, though, was was more adult and sophisticated. As much of a diehard Bangel shipper I am, Angel/Darla is pretty high up there in terms of my favorite dynamics. Julie Benz owns this season, and she and DB have some great chemistry together. Wolfram and Hart was also at their best with the perfect Holland/Lindsey/Lilah combo, and the writers went all out in showing was a brilliant character Angel could be. But like others have said, the season peters out after the Darla arc is wrapped up. I still really like the Pylea episodes, but maybe would have liked it more if they had been moved towards the middle of the season and the Darla/Drusilla/Lindsey arc came back full force for the final episodes.
So my final verdict is...I don't know. They both have their strengths and drawbacks, so I think I will consider them tied.