I'm not exactly sure which was one was literally better on, I think he's a great actor on both shows.
Though, Angel was his show and he obviously played a more obvious and more important role within the series.
On Buffy however, he played a very important part in season 2, being the big bad pretty much. I think he's a good actor on both and it's hard to decipher his best role.
Some may say Buffy, some may say Angel for some obvious reasons like because it's his show and because he was on for two more seasons than on Buffy.