I remember Angel commenting on her strength after Spike suggested both of them dragging her away from The Immortal.
I'm pretty sure we've seen her knock down steel doors, punch through concrete and lift steel girders. She's not the Captain America level Knight says she is IMO.
a heavy motorcycle overhead, a large fallen tree overhead, ripped the arm off of a super durable robot, carried a large telephone pole with a transformer attached, has broken steel doors in similar manner to Buffy, pulled down a helicopter, etc.
Buffy has more feats breaking through brick walls and such, and many of Cap's 'feats' are with the shield, but he has some that are in the same ballpark as hers, though his are a lower percentage of total showings IMO...
I'd put Buffy as stronger, and Cap about on par with Angel, and how much stronger Buffy is is hard to say...