Well surely, their value is all about the position they're in but i think bishops are slightly preferred over knights for a few fundamental reasons. there are some reasons that players "usually" doesn't want to exchange their bishop for opponents knight. while at the beginning or in closed position at middle game, players usually still want to remain their bishop because of the general conception that they will be very valuable at the endgame.
First, they are giving strength to a spesific color complex. when you lose, i dont know, white color bishop, you do not only see but also feel that you are very weakened on white squares. opponent's white color bishop remains unchallenged.
of course the bishops has the ability to go from one side to another side of the board in one move. this is a crucial thing at endgame. when it's centralized it effects more square more area. for example a game i played a few days ago for a good and bad bishop.
http://oi60.tinypic.com/6gbk49.jpgin the position white bishop is definitely a bad bishop and does nothing except protecting g2 which is pinned by black bishop. otherside white's very strong centralized bizhop not only pins g2, also creates the virtual mate thread on g2 if white bishop ever dares to move. also it protects f7 for any mating attacks, blocking d line for the passed pawn, protecting backward c6 pawn and does absoloutely lots of things. now value of the pieces should be considered for position as we can see here. white bishop is less valuable than black bishop so both example on the board.
otherside a knight may be can't do a lot like bishop on endgame but, well there is such a thing called "octopus knight" and family fork. they have different abilities which should be considered always. if other way well, bishop and knights wouldn't be counted as 3 point each surely.
and final thing if we expand the conception for two pieces, a bishop pair is clearly superior for a knight pair for me.