For me basically if you:
- Have the will (and sometimes the monthly quota $$) to store your music on the cloud via one of the many available services, AND have the connection to support it AND don't care about ratings, playcounts etc so much or at all.
- Only have 1, 2 movies TOPS on your iPhone all the time.
- Like podcasts but keep a few and after you listen to one you delete it right away.
- Have a few small apps.
- Don't care about storing at the same time more than around 1 hour of HD video.
Then I guess 16 GB is enough.
For me none of the above apply so even 32 GB is too cheesy. 64 GB all the way and wished we were in the future where 256 GB were normal.
Now, I've learned to rate music constantly and sync only a smart playlist with the criteria "Rating = 5,4 or NULL". Also I delete podcasts (specially video podcasts) right away after watching, and I'm constantly synching photos and videos to the PC (don't use iCloud Stream because I didn't like it).
Basically, for the 80% of human population, the 16 GB is just fine. Most people I know even buy an iPhone and 6 months later only have 2 or 3 apps installed, so...