The points system of 1 point for a pawn, 3 for a bishop or a knight, 5 for a rook and 9 for a queen is a useful rule of thumb, but nothing more. If you start making decisions based on a bishop being worth more than a knight you're going to end up with some lousy positions where you're opponent has two wonderfully placed knights in a closed position where your two bishops have barely six squares to play with between them.
Instead of this, try to work out for yourself when a bishop is better than a knight and vice versa. The books will tell you stuff like bishops are preferable in open positions and knights in closed positions and that bishops are better than knights in endings when the position is "wider" (i.e. pawns all the way across the board), and whilst this is a good starting point it is no more than that, I've seen thousands of games where the "bad" piece suddenly became good or where two knights ran riot over two bishops in an open position. The bottom line that is tactics rule - if your opponent has a knight on the edge of the board which is helping with a forced mate in 2 then it's a very good knight indeed!
It also depends on the player. You might find that you simply prefer one piece over the other. You might not even realise this yourself - take a look at games which you lost from what looked like equal positions and see if there's a common denominator, e.g. perhaps your not very good at getting your knights onto outposts, or perhaps you fall for knight forks a high percentage of the time. If so, exchange your bishops or knights as appropriate as early as you can!