In practice it has been shown that a bishop is usually stronger than the knight because of its mobility, but there are always some exceptions. Do remember that in such endgames position of the kings and pawn structure matters a lot. Here are some general pieces of advice with instructive examples:
The pawns are both the side and the position is more dynamic – the bishop is usually better
The pawns are on both sides and the bishop has no targets; generally the knight is better
If pawns are on just one side of the board and position of the king is not passive, then usually a side with a pawn down holds because the defender can sacrifice a piece for a pawn or pawns and can achieve an easy draw. We can conclude that if there are pawns on one side only, most of the time the game ends in a draw.