The web site tells us that the winner played Chopin and Debussy, but no more. It has more detail on the 2005 competition. The winner of that (Sabine Vinck) played "a Clementi sonata" and Prokofiev's Sonata No 1. We also learn that another of the finalists (Tim Morris) was the crowd's favourite and played Liszt's "Funerailles".
I'll be interested to see the answers to this, because I was planning to enter the next one in 2009, but you only get a 15 minute slot - which is a bit limiting. Beethoven did not write many sonatas that short. I can't decide whether to choose something flashy - some crowd pleaser from Liszt like a Hungarian Rhapsody or the first Mephisto Waltz (with something short as an opener), or a Classical sonata from Mozart or Haydn.
The fancy octaves and jumps look the more difficult, but when it comes down to it a classical sonata demands more control, and is a more dangerous choice, because a tiny mistake, as simple as coming down slightly heavy on a single note, can sound dreadful. With Liszt half the notes can "end up under the piano" as Artur Rubinstein used to say of some of his recitals ... and it will still sound OK!