Two bishops can checkmate a lone king. Two knights cannot. If the bishop in not blocked, the bishop is better. It covers more squares. It can cover as many as 13 squares. The knight covers 8 in the middle of the board and 4 squares on the edge of the board. Put a knight in the corner and it covers 2 squares. Put a bishop in the corner and it covers 7 square.
There is an age old question...which is better, a knight or a bishop? Personally I think the knight is best because it can move to any square on the board and allows for forks, but what do you think?