In general, a bishop is objectivly better.
Bishops likes open positions, since they are long-ranged pieces. Knights prefer blocked pawn structures, where they can jump over the pawns. But never underestimate a pair of knights working together. It's more difficult to make knights work together than make bishops work together.
In endgames, bishops are generally better if there are pawns on both areas, and/or if the opponents pawns are blocked on the bishops colour. With pawns on 1 side, the bishop can't really use the fact that it is longranged, but it has the handicap that it only has 1 eye. It can only use 32 squares, while the knight can use 64.
Bishop take knight is the small Exchange sacrifice (bobby fisher)
need to know :)