Compared to the normal version, the extended version is late story-wise, but it got sounds, voices, musics, and extra scenes (and the replay gallery I think, I've never played the normal version so I don't know if normal version also has the replay gallery but the extended one certainly does). Anyway, which one you want to download depends on your preferences.
We need a good quotable message that explain better than me so anytime someone asks the difference between normal and extended version we can just quote it.