Ai ai..!
Pretty hard to compare..
Fable is a third person action game with some scripted 'moral choice' moments and a variety of options on what weapons and tricks to use. It's pretty and has some interesting dynamic features but the combat is repetitive and the game has the depth of a spoon. It's story is also entirely linear, bar those afore-mentioned moral choice moments.
The Witcher is a well-written and atmospheric adult RPG, best compared to Fallout, or Baldur's Gate 2. It lacks the character customisation of either game, but the story and the motivations behind your quests, along with a wide variety of side-quests that allow a surprising degree of freedom in how you treat them, and a beautiful world.
Think of it like Neverwinter Nights as it should have been, the same small, detailed RPG world with a semi-linear progression between locations, but with fantastic writing, beautiful graphics, a mature setting, elfsex and decapitation.
Fable.. Fable gets tedious when you realise the entire game consists of hitting monsters and your moral choices do nothing but make different cutscenes happen. The fun is in people being scared of you or loving you, and abusing them emotionally and physically because you were sexually abused yourself as a child.
It would get a lot more tedious if you couldn't complete the game within 10 hours.
Two heavy sittings will finish Fable. It's disturbingly small.
The Witcher on the other hand is deeply engrossing but also grows tedious if you get bogged down in combat. My advice would be to keep a good amount of cash savings so you don't end up trapped in an area for a long period hunting and gambling for money when you need it for a quest.
It is, if anything, too long.
So.. Fable? A silly, british action game with character customisation and a sort of dynamic politics system where people will love or hate you, as your actions define your good or evil status, also altering your appearance. When it claims to be an RPG: It is lying =>
The Witcher.. Like reading a cynical and amusing dark fantasy novel, except you get to choose the path, it looks damn pretty and you cut people's heads off. A fine RPG in the old style, only marred by being exceedingly long, and having tedious sections if you find yourself aimless or low on cash. Stick to the main plotline, don't try to draw things out, keep a sensible amount of cash saved and it is truly fantastic =>