The cost would be roughly the same. ~$140 for either a 2696v3 or a 2699v3. Both 18 core, 36 threads. I think they're basically the same CPU but one has DDR3 support and one has DDR4 support. The decision would ultimately come down to if I feel like going super budget and getting this cheap board + the RAM I already have or buying a more reputable board and purchasing some DDR4. That difference is only ~$200 in cost, but when the entire upgrade is only going to be $350-$650 depending on parts, that $200 is a significant percentage of the overall.
The same question I have about performance persists when looking at the mainstream E5s though. I quite literally have no clue if an E5 + a P400/P600 is more or less performant than an i7-10700. I mean, shoot, the 3rd upgrade option for me is to donate the Ryzen 1700X that's in my desktop and spend my upgrade money on a new CPU + mobo + RAM for my desktop. But, again, it is difficult for me to discern unRAID + Plex + docker performance differences between these CPUs of different generations.