That always seems to be the million-dollar question when a new iPhone is released. It really comes down to which data plan you currently have. If you're on an unlimited plan, then I'd stick with 16 and stream all the music/movies/podcasts your heart desires. If you're on one of these ridiculous tiered options, you'll want to cache your music library (i.e. Spotify offline listening) and anything else that is downloadable via WiFi. In that case, I'd go with 32 or 64 and roll back your calendar to 2007/2008.