Aloha k,
I believe you meant ‘64 bit’ and if you are getting a new Mac pro,
it will come with Yosemite (OS 10.10.x) installed but if you do a Goog search
you will see that there are ways to ‘roll back’ to previous versions of the OS.
Personally (other than a niggling start-up prob, with C8)
I find C5.5/C6.5/C7.5 and C8Pro and Yosemite to be very stable.
Good Luck!
{‘-’}