These pieces are equal in value to me. The reason for this would be because the knight has a direct check and the bishop can work from further away...the downside of the knight is that he must be far advanced into the sixth or seventh rank in order to be effective. The downside of the bishop is that he can only control white or black squares, and it requires extreme skill to checkmate an opponents king with just your bishops and king, although I don't think that it is possible to checkmate with two knights...the combined effects can always come in handy.
Bishops or Knights

These pieces are equal in value to me. The reason for this would be because the knight has a direct check and the bishop can work from further away...the downside of the knight is that he must be far advanced into the sixth or seventh rank in order to be effective. The downside of the bishop is that he can only control white or black squares, and it requires extreme skill to checkmate an opponents king with just your bishops and king, although I don't think that it is possible to checkmate with two knights...the combined effects can always come in handy.
It is possible to mate with 2 knights and a king vs. a lone king, but it takes a stupid opponent to let it happen. This endgame happened in a Karpov- Korchnoi game, and they agreed on a draw.
Personally I prefer knights simply because I understand how to use them better than bishops. As I learn I'm hoping to use my bishops better and so balance their respective uses. After all the pieces work best together not as separate entities.
Bishops are better. Why? You can laugh at your opponent if you have the Bishop pair. No-one bothers about the Knight pair.

Two well placed knights can be really nasty, but in general the bishop pair is more desirable.
On an individual level it all depends on the position.

Knights in the Opening and Middle Game because it's cluttered then and Bishops are apt to be severely hindered by Pawns. Knights are also good for forking, and their method of checking without being able to be blocked are two skills that also come in handy.
Bishops are better in the Endgame though, because they can get across the board a lot easier. Also, I find that Knights seem to have trouble getting into position even once they're at the right place in the Endgame.

Bishop pair wins in middle or endgame over a knight pair. But I'd take a pair of either over a single bishop and single knight, because a pair helps to cover each other's weaknesses and create specialized attacks. In terms of having just one piece, I would rather have a knight in endgame, because it's easier to pick off single pawns or block several. In middle game, a single knight is good for blocking, (especially in the center four squares) but a bishop is better for attacks that are longer range, especially if queens are still on the board.
In the end, I think I would pick two bishops, but if only one piece was available, I'd go with a knight. I like some creativity in my games.

It all depends on the position. If having Knights gives me an edge and are key pieces, I prefer them. If bishops are giving me an edge, and again are key pieces, I prefer them.
Ideally I enjoy the company and resources of both at the same time!
Which one of these two do you prefer?? Also include why.