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I have always kept my 16GB full. The 32GB, not so much. However, this will have a larger megapixel camera and HD recording, and books, so your mileage may vary.

Most of us will have at least 1.5GB for a navigation app, and some of the cool games are getting into the hundreds of megabytes.

If you have the grandfathered unlimited data plan, you could get away with streaming music and videos from SimplifyMedia (if you still have that), the multitude of radio apps, and SlingPlayer and Air Video, and therefore depend less on filling up the iPhone with music and videos all the time.
 
I'll be getting 16GB model for sure. I'm streaming pretty much all of my media from my home server or the internet so I don't really need extra space in my phone. Especially now iOS 4 permits background music from 3rd party apps, I think I'll be just fine with cloud based media. Even after putting Navigon data, some "backup" music and movies I have plenty of room left on my iPad 3G 16GB. I was waisting the empty space big time on now-gone 32GB 3GS.
 
I will personally be going with the 32 for a couple of reasons.

1) I have a reasonably large (25ish gigs, hoping to trim some of the fat off of it) library of music. My ipod video is years old, and I'd rather spend the extra 100 now than buy a new ipod at 250-300 dollars when it dies (which will probably be sooner rather than later). I'm also the type that likes to have all of my music with me - making small playlists for the gym / whatever else just doesn't cut it for me.

2) It will be my first time owning a smartphone, so no grandfather unlimited data plan for me - which means a limit to what I can stream.

3) 720p video recording
 
Are people willing to pay the same $$ for a Used 16g and 32g?

What I mean, is the resale (used in a year or so) worth paying the $100 upfront if you may NOT need the extra 16G?
 
Are people willing to pay the same $$ for a Used 16g and 32g?

What I mean, is the resale (used in a year or so) worth paying the $100 upfront if you may NOT need the extra 16G?

I think the price difference between 16 and 32GB in used market won't be $100. I did a quick quote at Gazelle. Perfect condition 16GB 3GS gets $226, and 32GB gets $304.

I think the price difference of two models also scales down as the resell value shrinks over time.
 
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