16GB is the bare minimum unless you plan on only using it as a phone. 8GB is just torturous. The file sizes of apps are ever increasing as well. Put a couple of those on there, a handful of pictures, and some emails...you'll be playing Sophie's Choice with your media very quickly.
32GB seems to be the sweet spot for the average Joe who likes to keep their music library, a decent collection of apps, and pictures without having to clean it out every week.
In the end though, it's all about how you'll use it.