I'm having the 32gb iPhone, I chose that one specially because of the high definition video capabilities on the 4S, my previous iPhone 4 was a 16GB and the storage was filling up way to fast because of videos taken with the iPhone, yet the iPhone 4 is a 720p so when I think about the videos size of the 4S I decided that a 16Gb couldn't be a good deal.
If you are not about using the iPhone to be making videos, or sporadically making videos, then go for the 16gb. It's way enough for apps and SMS iMessage, and even gaming progress saving, unless you are planing to load 10gb of music on it then you should go for the 32Gb.