Is it better losing a knight and a bishop or losing a tower and a pawn? I'm wondering? What do you think?

would you rather lose your flashlight or your first aid kid? it totally depends on the situation...

It's better not to 'lose' anything! (usually anyway)

Erik is right.  Piece value determines the strength of the piece at the start of the game.  As a game progresses the pieces attain actually different values to what they started with.  For instance, a pawn that's about to Queen is a lot more valuable than a pawn that is still on your second rank, but both are still pawns!  This is a simple example but exactly the same principle applies to every other piece.

NOTE:  Material is not as important as the position, greater material usually equals a better position but not always.