Help with CPU selection

Neither one is ideal. Both of those have been surpassed by newer CPUs from both Intel and AMD. What's more, that i7-7740X is only a quad-core CPU whose performance in Premiere Pro is BARELY any better than an i7-7700k that's now over two years old. And not only is it only a quad-core CPU, but it only supports dual-channel RAM as opposed to the quad-channel RAM of the higher-end CPUs.

Plus, you will NOT be able to utilize Adobe's hardware H.264 encoder at all with either CPU since both completely lack any integrated on-CPU Intel HD Graphics. You will be permanently locked to software-only encoding with either of those two CPUs. (Do not confuse hardware H.264 encoding with the MPE renderer; the latter uses CUDA or OpenCL as opposed to Intel QuickSync.)

And I would not spend much if any money for an X99 chipset-based motherboard that would be needed for that i7-5960X since it is already obsolesced, and would become less and less likely to remain supported as time progresses.

In other words, if either of those two is all that you could afford, then you would be pretty much stuck between a rock and a hard place.