The Witcher or Shivering Isles?

Now that I'm done with Mass Effect, I'm torn on which game I should tackle next. I don't think I'm even a quarter the way through The Witcher. Like Mass Effect, that game tells a great story, though not quite as polished as Bioware's sci-fi epic. I finished Oblivion a long time ago, picked up the expansion, Shivering Isles, but barely explored its juicy content. In addition, I want to continue side-playing Mass Effect; my new toon looks like Gwyneth Paltrow and she's mean as hell. She's already made three NPCs break down and cry.

This is a good problem to have but I have to fight the urge to try and juggle all three at once. That's what leads to not finishing any of them. I think The Witcher gets the nod because I've got more of it to finish. On the other hand, I could knock out Shivering Isles, which is shorter, and continue playing Gwyneth Paltrow in Mass Effect.

What to do, what to do.