Yeah space consumption seems to grow with availability at thresholds. What I mean is, when I had an 8gb phone I was like... okay that's fine, I'll just make a few playlists of my favorite stuff and hardly ever used over 5gb.
Then when I had a 16gb phone, similar situation but I filled it up easily because I started putting unwatched back-episodes of The X-Files on there, along with every daily show and all that. Still I didn't care, I could've done with 10-12gb and just carried fewer episodes.
Then 32gb comes and magically I can fit my entire "checked" music collection at once, several movies, tv episodes, and all the apps I can eat, including the big adventure games. But now I have like 1gb free and some days get the warning about filling it up because I forgot to watch an episode of TDS or something.
So the bigger my available space was, the more easily I filled it ... go figure. And to add to the point, other than apps, the vast majority of which are small, I had all the media before the phones, so the greater space didn't drive me to buy a bunch of TV episodes or something.
Ultimately I think the only real advice is to... buy as much space as you can afford to. You can't predict your habits until you can practice them with the available storage. If you're even thinking about running out of space filming stuff at 720p, then don't even consider the 16gb, just save yourself the trouble and get the 32gb, it's only $100 more, and the flash memory in these devices is quite fast. As a comparison, class 10 sandisk 16gb SD cards cost about $95 on Amazon (can't risk counterfeits when performance counts). I would estimate the iPhone flash is between class 6 and 8, meaning that even if you could buy more memory later, you wouldn't be saving that much by doing so.
But you can't. So just get the 32gb :-D
