Just a note before anything else: it's not a "manufacturing process". Nobody is laying out the transistor by hand, it's a fabrication process. (I apologize for nitpicking in the wording, but it's important)
Apple does these kind of things for two main reasons: Because they have the money and because they want to test both fabrication processes for future products
The differences between a 14nm process and a 16nm process are quite minimal mostly because one process can offer some advantages over the other one. For example: it's expected that the smaller process has bigger leakage current, increasing power consumption, while it's expected for the bigger process to produce more heat.
In the end, you could say that if you sum the advantages and disadvantages of both, you will not reach any conclusion if you are not the engineering team looking for extreme optimization and with Apple's resources at your disposal.
It's just hard to come to a concrete conclusion from a consumer's point of view.
