Hi, the two aspects “64 bit machine”(OS actually) and “64bit project setup” (Audio processing) have practically nothing to do with each other. The first talks about adress-spaces for the OS and some other technical details.
The audio processing (64bit project setup) affects plugins as aleady said and the “precision” of audio processing. There is serious doubt that (except for some plugin-types) that there are any relevant differences in audio quality resulting from this. As technology evolves it has become a habbit to exploit it, even though audible differences are questionable. This is very much like the 8K television-thing.
Bottom line: The two aspects are not linked. If you are convinced to benefit from 64bit audio processing and your machine can handle it - just do it. If you are not convinced, stick with lower values - it will save processing power and space.
Going to a 64 bit OS is more or less mandatory, 32 bit OS will extinct.