All i've ever heard about polyphony is that the more you have, in general, the more realistic it's going to sound. I recommend basing your consideration on price and sound vs what could potentially just be more of a gimmick for you.
16-bit audio represents the spectrum of what we as humans can hear. However, as soon as we start manipulating 16-bit sounds digitally we begin to have unnatural sounds... every rounding error is within that spectrum of sound, which means that if we have good enough ears, we can hear every bit that went wrong in the digital computation process, whether that process is reverb, compression, whatever.
So, technology bumped up to 24-bit recording. The level of detail is so fine that you can't hear the most subtle parts of the sounds now..... those extra 8 bits (of "depth" as the pros call it) mean that you can have up to 8 bits deep of rounding errors, smearing, or even just extra detail. This improves the "audible" portion of the 16 bits we do hear.
There is no 64-bit recording. There isn't even 32-bit recording. You can only hear 16-bits worth of sound...