> They are entirely different sensors, and share nothing at all in common.
I wouldn't say that. The 5s uses the IMX145 and the Nexus 5 uses the IMX179. Both are Sony CMOS arrays.
The 1/3.0 size of the 5s array does enable larger individual photon 'buckets' for better light sensitivity but they're both closely related in terms of contemporary Sony technology.
The faster 5s lens is a key differentiator, I agree. But it is rather outclassed by the the Galaxy S5 and the Xperia Z2 / Z3. No serious Android-using photographer looks much beyond those for a smartphone.