Bishop or Knight?

first

No preference. Depends on the position. A centralised knight is a fantastic beast but soooooo slow on an open board where bishops rule.

one of my friends used to say that a lone bishop is a pathetic thing  

its a complex question.  knights can hit both color squares, but can't cross the board fast.  Knights can defend each other.  Bishops are locked on one color but can cross the board fast and work well with pawns.  Knights can move around enemy pawns, bishops can get locked up behind their own pawns or unable to penetrate the opponent's wall.   Bishop pair is extremely powerful, knight pair apart from mutual defense is sort of meh.   And, it heavily depends on what is left on the board along with the position.  Early game, if you can break a bishop pair for a knight, that is almost always an advantage.   Late-mid-game and endgame, the position trumps any rule of thumb stuff though.   And trading early generally if it messes up the opponents pawns is good -- many a player will trade a bishop for a knight to double pawns.   Also good/bad bishop can be a decision maker if you play openings that lock one up. 

Closed position, knight. Open, bishop.  Also in the end game bishop v knight, knight is better because you can put all your pawns on opposite colors, or even block the bishop. 

Hi!  My name is Lauren Goodkind and I'm a chess teacher based in California.  

If there are several pieces on the board (not a lot of pieces on the board), then the bishop is better.  If there's a lot of pieces on the board, then the knight is better since the knight can jump over pieces.  

I hope that this helps.