Oblivion GotY edition...?

I played the Knights of the Nine add-on, and it was ok. It was pretty short, and too easy and repetitive for my taste. I haven't played Shivering Isles yet, but my cousin was obsessed with it, and it was all she did for months, so it may be worth it. Also, If you sold your original version and then bought the GOTY edition, you probably wouldn't be out too much anyway.