In this post form Forbes, it suggest that Apple made the iPhone more expensive even though Apple did not raise the price one penny (not the US prices - prices have increased for some countries as the USD has strengthened over the last few months and may continue to do so as commodities market collapse and some countries like China manipulate its currency). What Forbes suggest is that new features like live photos (think Harry Potter but you do not have to be a wizard) and 4K video recording would require additional storage, making the 16 GB models of the 6S and 6S Plus pretty useless as apps, videos, and photos fight for space on the smallish storage capacity.
This has been an issue but it is only more apparent with the big jump in camera capability from the current 8MP to 12MP on the iSight camera while the 5MP selfie camera on the front will force selfie-takers to consider what pictures to keep. This would require no only additional storage per picture but with live photos, it is twice the amount storage a regular photo would take.
Furthermore, 4K video could well mean that users taking even casual videos will need to be aware of how much storage they need - figuring into whether to stick with the low-end 16 GB model or opt for the 64 GB or even 128 GB models.
Which brings up the question: when will Apple release a 256 GB version of the iPhone, and eventually, the iPad as well. And should Apple actually offer a 256 GB option, will Apple continue to offer the pretty useless 16 GB or finally let it happily retire into Apple history?
That speculation is entirely perfect for a post unto itself. Simply put, while Apple will not come out and say it, it's obvious the middle tier 64 GB iPhones are the bare minimum if users take any decent number of photos and videos. To be safe, the best bet is the 128 GB.