I think the 16GB RAM makes more of a difference than the i5 or the i7. I have the i7 but I noticed a massive difference following the recent upgrade to LR - I understand that it now uses the GPU rather than the CPU for most operations.
Since most operations in Lightroom require a fast CPU, I would get the fastest CPU you can afford.
There are many web sites that perform benchmarks of different CPUs, and also rank the CPU according to speed per dollar spent (or some similar measure). While none of these benchmarks is specifically for Lightroom, I would say it is still true that the fastest CPUs will perform better with Lightroom than a slower CPU.
I understand that it now uses the GPU rather than the CPU for most operations.
The GPU is used for SOME operations in the develop module. It is not used outside the develop module. Furthermore, according to Eric Chan, the biggest improvements in speed from using the GPU will be seen on 4K and 5K monitors, and if you have an ordinary 1080p monitor, you will get only minor benefits from the GPU, and you might actually be better off if you disable the GPU acceleration in Lightroom. See GPU notes for Lightroom CC (2015)