I'd recommend the 32 if you have the money. Although the 16 is plenty for me, I'm conservative of my iPhone's storage space....
32 seems to be perfect for most people especially if they plan to have games, music, videos, podcasts or any other median on their device.
Most likely you'll need more than the 16GB or you will come very close to meeting 16GB... I'd go for the 32 as I doubt you'll need more unless u think you may have tuns of videos or music on it.... 32 is enough for more than 2000 songs