By Vader saying "no" it takes away any emotional impact. Originally it was between loyalty and love, and Vader chose love but not easily. You could tell he was conflicted without words or without seeing his face. It's a powerful scene, and that's why it's my favorite scene in the trilogy (along with the fight that occurs before it).
With the inclusion of "no" it transforms the scene from an internal conflict to something that feels like it's spur of the moment, like he didn't think about it, which is the total opposite of Vader's character.
I'm not going to dl it, I'm going to get the dvd versions of the theatrical cuts. They're OoP so I gotta act fast.
Anyways, my favorite part of Star Wars, including the Emperor torturing Luke:
[youtube=560,345]HS1mr2ty8bU[/youtube]