For CPUs it's normally quite easy to decide which hardware performs better and is more cost efficient. You could actually just have a look at some benchmark sites and choose a OpenCL-compatible CPU that is worth its price (e.g. from benchmarks from cpuboss.com or cpubenchmark.net or any other benchmark site).
I think that only modern high-end Intel CPUs are currently recommended if we look at the performance (modern AMD CPUs, as far as I know, do not even support a modern instruction set like AVX2 or even OpenCL as good as Intel CPUs do).
The problem is that the most recent version of Intel CPUs are quite expensive (i9 etc), but of course they would also be much faster (see benchmarks to make sure about the individual performance and how much the speed increase is).
I think with a good/modern Intel CPU (yes even Xeon, if it is compatible with a "cheap" motherboard, and worth the price) you can't be wrong (even if they have some serious security bugs
see meltdown or spectre).
Unfortunately, if we look at GPUs instead, with -m 15700 you currently could use only something like a very high-end Nvidia 1080ti GPU (at the time of this writing) and the performance might be only slighlty better than a mid range CPU (or in some cases even slower, because of the high memory requirements and therefore high tmto values and multiple memory allocations needed, because of restriction on how much memory can be allocated at once). I'm not sure what the current performance of -m 15700 is with a 1080 ti, but you could just test it or look for benchmarks.
Anyways, scrypt is known to be GPU-unfriendly and therefore a modern OpenCL-compatible CPU will be a good idea to crack a 15700 hash (what might be even better is to have a good strategy on how to attack the hash: a plan to generate/use the most likely password candidates... a "small" set of candidates of course, because scrypt is slow... but this is of course a different topic).
I think that only modern high-end Intel CPUs are currently recommended if we look at the performance (modern AMD CPUs, as far as I know, do not even support a modern instruction set like AVX2 or even OpenCL as good as Intel CPUs do).
The problem is that the most recent version of Intel CPUs are quite expensive (i9 etc), but of course they would also be much faster (see benchmarks to make sure about the individual performance and how much the speed increase is).
I think with a good/modern Intel CPU (yes even Xeon, if it is compatible with a "cheap" motherboard, and worth the price) you can't be wrong (even if they have some serious security bugs
see meltdown or spectre).Unfortunately, if we look at GPUs instead, with -m 15700 you currently could use only something like a very high-end Nvidia 1080ti GPU (at the time of this writing) and the performance might be only slighlty better than a mid range CPU (or in some cases even slower, because of the high memory requirements and therefore high tmto values and multiple memory allocations needed, because of restriction on how much memory can be allocated at once). I'm not sure what the current performance of -m 15700 is with a 1080 ti, but you could just test it or look for benchmarks.
Anyways, scrypt is known to be GPU-unfriendly and therefore a modern OpenCL-compatible CPU will be a good idea to crack a 15700 hash (what might be even better is to have a good strategy on how to attack the hash: a plan to generate/use the most likely password candidates... a "small" set of candidates of course, because scrypt is slow... but this is of course a different topic).