It's been several years since I've played either, but from what I remember the atmosphere in the two games are completely different. Witcher (the 1st one) has far more of a 'realistic' dark ages feel to it. As for the combat, from what I remember, Witcher has a lot more depth, with an entire alchemy component thrown in for preparing oils to coat your sword with. The oils give your weapon bonuses against certain types of creatures. In fact I would suggest playing the game on hard, if for no other reason that even on normal difficulty it's never necessary to prepare any oils to defeat monsters. Fable by comparison has far more standard approach when it comes to combat. You can go the magic route or you can go the physical combat route, with the latter split into quick + light armour and slow + heavy armour. Again, it's been years since I played so I might be off on the details.
Both games are kinda semi-open world, and if memory serves you can visit most places again in Fable even late into the game. Witcher has a kind of chapter approach, with each chapter consisting of a bunch of large areas you can traverse freely, but these get closed to you when you move onto the next chapter/section. I remember being extremely upset when I learned I couldn't go back to a certain windmill in the beginning to meet with a fair maiden. She said I had to bring a bottle of wine, and I figured I could progress the story to enter the city where I would surely be able to find some wine. Sadly that constituted a new chapter/section and I never saw the windmill again :(
Of course Witcher is also more, uhm, mature :)