I'd buy what we have again.
For my 14yo, the 32gb wifi. She is oddly using it as her music player, and has our entire music library on there, whish is like 13gb. Then she fills any space left with movies. She could, like any teen probably, fill a 64 in a flash. But the 32 gives her "play space" and "school space". It was a good choice. (she has an 8gb nano, the 3rd gen one, which is where I expected her music to live... She uses the iPad the most now though)
For me, I'd buy the 32gb 3G again. Gives me flex room to load it up with school stuff for the younger kids, and have own stuff. Sure I would have bought the 64 if money hadn't been a concern.
I have a 32gb touch and it has worked out better than I thought. I knew I'd still be using it for music (which has my library on it), and offloading my mail & PDFs to the iPad, so the 64 between the 2 of them is really about perfect for now and let's me just not have to think about it.
16 for me would just mean more work. I don't want to have to juggle things around that much. The experiment with the 8gb touch showed me that in the 6 months I had it before passing it to a kid when I got enough money at Christmas to upgrade.
As for the 3G, as I have stated before, I'm out of the house at kid things 8 hours at least a week, with an additional 3 hours of travel time. None of those places have wifi. It's a big chunk of time, and this week I actually got a project done while they were at art class. I didn't have to come home and get sidetracked by another project, I worked on what I had with me. It was nice. I should add here, I don't have a phone with data, so this replaces that in my budget.
All in all, I'm beyond pleased and have No buyers remorse for bigger or smaller.