You can mate with 2Bs. You (generally) cannot mate with 2Ns.
Bs can move from one side of an open board to the other much quicker than Ns in the end game.
So generally in an open ending 2Bs are to be preferred.
In a closed position it could be that the Ns are better as they can reach every where when a B might be restricted. If this can be converted into a material advantage then the Ns could win.
... but generally I would rate Bs as worth fractionally more than Ns and they should only be exchanged off if there is no alternative or if you create other weaknesses in your opponent. Doubled and / or isolated pawns can often be a sufficient weakness - the difference is that marginal.
Weaker players often struggle against Ns because of their ability to fork and for this reason often perceive Ns as being a better more powerful piece and whilst it is true that a N fork can be a dangerous tactical threat experienced players are aware of it and modify their play appropriately.