I have hard time seeing iPhone last more than 3 years. That is 36 months, or $360 of spotify subscription. Now price difference between 16GB and 128GB for iPhone 6 is $200 before tax. This comes roughly just under two years and is about average interval tech savvy people update their phones at.
This is just putting things in perspective, unless you have very specific needs of your phone I do not think 16GB is enough anymore, 64GB is definitely way to go.
I use my iPhone a lot - blowing through 64 GB is pretty trivial. It's not even clear to me that 128 GB will get me through the full two years, but, at least I won't have to spend all my time deleting / re-downloading music. And, might be able to keep a few videos (I've had to delete them all off my iPhone to make space).
I spend a lot of time traveling overseas, and have been hit by more than a few >$500 phone bills, even with the international data plan and lots of use of WiFi Dongles+Local SIMs. The more data, maps, podcasts, music I can store locally, the less use of data required.
I probably won't be happy until we get a 256 GB iPhone, which, on current trends, probably won't be for another 3-4 years unfortunately.
To put it another way - 128 GB isn't enough for me to stop worrying about saving space on my iPhone, but it is enough that there is little value in me shuffling music on/off it.
This is just putting things in perspective, unless you have very specific needs of your phone I do not think 16GB is enough anymore, 64GB is definitely way to go.