Whether to keep a knight or a bishop

If I can go into the endgame with one minor piece should it be a knight or a bishop?  A bishop can range the full diagonal of the board in one move, but a knight can cover any square and weave among scattered pawns more easily.  I always seem to have the wrong one.

It depends on a lot of factors... you'd rather have BOTH bishops... very strong... but in general, you want active pieces... if your opponents pawns are all on white squares, it sure is nice to have a white squared bishop handy... if your knight is trapped behind your pawns, it stinks... a nice open board favors bishops IN GENERAL... A closed one, knights... when exchanging pieces, try to see how the game will look down the line and try to figure out which pieces will be best... once you find yourself stuck with one, try to make the board favor IT. 

Bishop is better!!!

http://www.chess.com/votechess/game.html?id=23251

Hehe, that's true.  Look at your games, why do you know your pieces is the wrong one?  What about the position do you think makes your piece bad?  And now you know what to look for :p

In very general terms, bishops like open games where there are things going on on both sides of the board.  Knights are generally better when most/all the action is going on in a relatively smaller area, or when pawns are only on one side.

Bisohps don't like their pawns blocking their movement, so if you have a lot of dark square pawns, I'd first see if I can go ahead and give up that dark square bishop.

Thanks for all of the comments.  I haven't seen any brilliant new theories but you have been coming up with pretty much the same things that I have thought of.

 Yes

Generally with mobile (not blocked) pawns on both flanks, and especially with a separated passed pawn, you should keep a bishop; with pawns on only one flank - a knight; with a blocked position with pawns on both flanks - the knight if it has a good central outpost square.