I have gone with the 32GB variant every year since the iPhone 4...64GB is useless to me because between iCloud and 32GB, I never even come close to hitting my max...that would present an option to the 16GB to some, but I have found that with all of my installed apps, even if I were to remove my music that is stored on the phone, I didn't have enough storage to feel comfortable with 16GB because of one thing, and that is taking a metric TON of pictures and videos.
This last baseball season alone we went to about 25 Braves games (and one Giants game, lol!)...I have about 200 pictures of JUST baseball stuff...about 20 of those are panoramic pictures, and sprinkled in there is about a dozen or so HD videos of various things at the ball park. That's just baseball...that accounts for about 20% of my pictures, lol.
With that kind of usage, and the fact that I never delete my stuff after I move it to my computer (for what reason I honestly do not know), I usually sit right at 20-25GB of used storage on my phone throughout the year. Taking into account the OS usage, I have about 5-7GB worth of free storage at any given time. I'm comfortable with that.