I went 32 GB mainly for the added space to put Movies on it for vacations, so I wouldn't need to bring the MacBook.
Also know that the Apps take up more space than you would think. I have 76 apps so about 6.4 GB in apps and games alone. Then under 4.5 GB in Music, I have most of my music on my iPhone 4. And the rest will be mainly for movies. I also have ebooks in the kindle app and magazines in the Zinio app.