The resale value is always higher on the largest capacity device, as well as the ease of selling it. If someone is looking for a 16GB, a 32GB will also work, but for someone who needs 32GB, 16 will not be enough. It's an extra $100 for the extra capacity, but resale value could be more than $100 higher depending on when you sell it.
I say stick with the 16GB, if you're not loading music or videos on to it, then there is no point in getting a larger model.
I myself have the 16GB, and hopefully will be getting a 32GB model, as the phone has now replaced my ipod, and 16GB isn't enough for me, I'm almost maxed out!
It depends on what you use your iPhone for. 🙂 I use mine for (a lot!) of music and applications, which means I use almost all 32g. If you only use your iPhone for the applications that come with it (Calender, calculator, iPod etc.), then I suggest you stick with 16g.
I went with the 32gb, I don't put a lot of stuff on my phone but I figured since it has video and I have a 2yr son, then I could justify getting the bigger one. And like others are saying the resale will be greater.
I have a 16 GB 3G and have 6.5 GB available, but I don't have all of my photos on it. I upgraded to the 32 GB because of the video capabilities and larger image sizes of the new camera.