There is much reduced hiss, thereby making the listening experience more pleasurable. This article attempts to compare and contrast the discs. Oh Mercy, with the extra songs listed at the end. All identical performances, identical mixes. Less hiss on Bells.
Bells has less hiss.
Less hiss on Bells. The latter works better. Note this is not Dylan choosing to sing it again because the piano playing and the singing are identical. I guess this was done because the cut had made the song much shorter but in reality it is pretty pointless. Bells has much less hiss.